As of my Reawakening to magick, all my personal workings have a 100% success rate. As of becoming a professional, all my paid workings have a 98% success rate. The one ritual that “failed” didn’t fail to do what I predicted it would do, it just didn’t meet the expectations of the client. I put that into the soft fail category, because the magick and the spirits involved did do what I knew they would.  
They didn’t give the client what they wanted, but they did give them what they needed! 
Otherwise, I can't remember the last time a spell I truly put effort into failed. When I DID fail at magick it was before I really understood what magick was. Oh, I knew the procedure, and I knew the mystical nature of it. But I had certain misconceptions, partly due to pop cultures (mis)representation of magick, giving me the wrong expectations. 
In the end, this is a big part of why I became a skeptic for 20+ years... after years of experiences that I could not deny, I eventually decided it was all just some kind of self-delusion brought on by my desperate need to overcome the insurmountable issues of my childhood and my own character flaws. I believed I was schizophrenic, or prone to hallucinations, and built up this whole fantasy world in my mind to explain things that were merely spooky coincidences, but nothing more.  
Like many who become interested in magick, I sought it, and the demons and shadows I communed, with because this made me feel powerful. It gave me a sense of control and made me feel like more than just some guy from the wrong side of the tracks with a difficult homelife who could barely hold a job. 
But at some point, I began to fail, after talking to demons and shadow beings, sometimes daily, for years, I broke down. I began to see less and less success... and between that and other things changing in my life, I walked away.  
(You can read more about this and it’s causes in my book SHADOWNOMICON) 
Trust me when I tell you this, I am not at all regretful, in all those years I became a STRONG person, I became self-aware in ways most people are not, I eventually reconciled a bit, and became a skeptical mystic, someone with a secular and philosophical interest in mysticism and spirituality, rather than a dogmatic, superstitious or religious need for it.  
I focused on enough material things that I was able to at least understand myself and no longer have these hang-ups that I had going into magick the first time. 
Magick for me now is no longer about coping with a shitty life... it is now part of my expression of how awesome my life is! I came to terms with reality.  
So, while I do lament the loss of time, time I could have spent educating myself about magick. I spent it instead mastering the tools of the skeptic, and understand better how real magick is, and why it may not seem like to everyone  
I know intuitively and from experience being taught by demons and trial and error what others need books to tell them. As I often say, I am not bragging, this is confirmed time and time and time again. 
Anyway, let me cut to the chase of why I failed and why most others fail, even if only sometimes. There are many reasons why people don't have such a success rate as I have now. But let me limit it to just 3. 
So, without any more distraction - Here are the 3 biggest reasons I used to suck at Magick and why you might too. 
1: "Improper" or "EXACT" expectations –  
People think magick is a vending machine that grants wishes, that you put in a spell working and you get out an outcome. Like a genie in a bottle that grants wishes. Instead, magick is much less mechanical than that. It is chaotic, it is fluid, magick is more like a force or influence, and that influence doesn't (normally) give you what you specify, so much as it gives you what you specifically need, or more accurately, it brings you what you intend. Which is not always what your own ego mask wants.  
Example - You hate a co-worker who makes your job hell! You hate them more than anyone has ever hated anyone in their lives. You want them DEAD. So, you stow decided to use some death magick and hurl a death curse at them with a one-month time limit for manifestation. You pour your heart and soul into it to the point where you aren't even mad anymore. You close the ritual, feeling cathartic and walk away, not dwelling on it, letting the magick do its thing. In other words, you did everything right. 
Result - nobody dies, but 2 weeks after working the spell, on a Friday, the person you hate is emptying out their desk with a big smile on their face, they got a last minute promotion because someone in another office got fired, and so the person you hate is getting a raise, and are moving to a bigger better office 2 states over, you will never see them, and they will never interact with you again for as long as you live. 
A blind fool or skeptic would consider that a "failed" spell. (And I once was such a blind fool.) 
While I would NOW consider that a rip-roaring success. (And now I am just a crazy fool) 
If I was that Sorcerer, I would realize that I ended up getting what I really wanted, peace. space, distance, and an end to the relationship. I said "Death" because I was being vain, and selfish and vindictive. But my true will wasn't for them to die, it was for them to leave me alone! 
You could even rationalize that in a manner of speaking they ARE dead, DEAD TO YOU! 
This is a big reason why some level of divination should be done before taking magickal action. Even if you don't believe in Tarot or Scrying, it can help you to make better, more honest decisions, or inspire you to think about things in a way you might not have done before. You might figure out how you truly feel, and then make your magick more aligned with your true intention. 
Also - People don't leave their spells enough room to accomplish their true intention, they don't do any divination and they are too specific, too demanding, too needy for magick to be like a machine that grants wishes. The best magick is somewhere between asking magick to do what it wants and asking for it to do what you want. 
Say you need 546$ to pay off the rent before the end of the month. 
You could cast a spell for 546, or 600$ or some other number. And never get it. Or maybe only get 230$ or 5$. 
But if you cast the spell so that the spell is really aimed at making sure the rent is paid or the landlord gives you an extension without any problems, without being specific about how much money or what path the magick has to take... NOW you are casting a spell that has room to breathe. Now, you might get 600$ worth of free car repairs from your daughter's new boyfriend as they try to kiss your ass, so you let them go away for the weekend etc. Thus, saving you the money. 
Or maybe you will get the rent extension, or maybe you will pull some overtime and make rent just in time. 
Either way Spell = success! 
2: People have "unrealistic" expectations of what magick can do – 
Another reason dealing with expectation - Now you need to know, I do experiment with magick, and part of experimentation is failure. But I do not consider experimentations to be a "success/Fail" binary situations. The experiment might fail, but my magick didn't. Most of my magick experiments are trying to learn something new or find the limits of magick as I understand it. 
For example, if you asked a sniper to hit a moving satellite in orbit, does that mean his shooting skills or his rifle or his aim failed if he can't hit it? 
No... It means the task wasn't rationally possible. 
This was, in fact, the NUMBER 1 reason my magick began to fail me, and why I fell more and more away from the world of magick becoming a skeptic as a result. The more I tried to do the impossible the more impossible magick began to be. The more I failed, the less I believed was possible.  
Eventually, even spells I knew worked had stopped working, it was like being on a lucky winning streak and suddenly the law of averages caught up to me, and nothing worked, and I mean nothing. 
It was like I was cut off...  
Part of that is because my belief in my ability and the nature of magick itself was rent asunder. 
I lost my mojo, and my confidence and my way. I fell off the horse…  
So, what is realistic and reasonable? Well, ask 9 mages, get 18457 answers. Everyone has stories everyone has those tales that just sound like the person is telling you some bullshit to sound interesting. Myself included.  
So, I can't tell you exactly. But I will tell you this. 
REAL magick follows a path of LEAST rational resistance. 
That means, the fewer miracles it must perform to do what it does, the more likely it is to work that way. It’s kind of like Occam’s Razor, which states that the explanation or theory which requires the least number of assumptions is the superior one. So, in the same way, the less unreasonable things magick has to do to bring you the thing you want, the better and easier it is to do.  
This is why it can be so important to do divination and meditation on a situation before trying to use magick on it. It can be important to see where the winds of causality and chaos are blowing to make sure you will not find all of reality in opposition to your goal.  
The wise woodsman splits wood WITH the grain, and saws or chops against. Magick is more like splitting wood than chopping or sawing. We seek a line of least resistance and apply spiritual intent and magick to crack open the path for us to get what we want.  
Therefore, Real Magick is not Fireballs, it's not people flying like Superman. It's not demons appearing in your closet to steal your children's souls and it's not the force. 
Real Magick is swamp gas that looks like a UFO. 
Real Magick is spooky coincidences that skeptics deny have meaning, but you know better. 
Real Magick is dreams that come true. 
Real Magick is that bump in the night caused by the neighborhood cat. 
Real Magick is always at the exact right place, at the exact right time, to have the experience of magick. 
Real Magick is that voice we hear in the back of our mind before we do something stupid. That nagging voice telling you not to do that dumb things! 
Real Magick is that Tarot spread that is just so accurate it's spooky. 
Real Magick is Subtle. 
Real Magick is mostly in your head and can be mistaken for imagination. 
Real Magick is things going missing or turning up. 
Real Magick is missing time/lost time. 
Real Magick is almost dying, but somehow not despite the odds. 
Real Magick is invisible to skeptics. 
Real Magick is EASILY explained away as something NOT magickal. 
- Every time. 
Except when it isn't ;) (all rules need exceptions) 
My point here is, magick CAN do almost anything, but it almost never does it in those impossible ways. When it does cause miracles, it can change the course of history, or does so where only a handful of people will see it, and never in a way that cannot be rationalized by outsiders.  
Casting a spell for 100$ is almost never, or at least to my knowledge has never, resulted in a 100$ bill appearing out of thin air into the hand of the Sorcerer. I won't say it CAN'T do that, because even science says that due to quantum probability it's technically possible, (just really really really unlikely - See Boltzmann brains). 
The number of times I have seen magick manifest in a miraculous obvious way, could be counted on 1 hand. Magick is far more likely to manifest in strange less dramatic ways. IE poltergeist activity, seeing fire take the shape of a being and hold it briefly, hearing strange sounds (like a crow call at midnight) just as the ritual was asking for a sign of the spirit to manifest). and seeing shadows with a group seeing the same things and stuff like that. 
This is another reason why some kind of divination can be helpful. Divination is more likely to give you solutions to problems and concerns that are rationally possible for magick to deal with. Or inspire you to find them yourself.  
3: A lack of understanding of the holistic nature of magick. 
To me, this is a pet peeve. I watch a lot of sorcerers do their thing on YouTube or other contexts, and I want to reach through my monitor or whatever and throttle them... 
Magick is not a collection of strange words, poetry, black candles and funny shaped called sigils or circles or whatever. 
Nor is it a purely imaginary thing you do all in your mind's eye. 
Nor is it purely a thing done on the astral level.  
Magick, real magick, is a holistic activity, it is literally a Yoga, it is a union of all that is, a practice of mind, body, and soul moving as one. Indeed, if I didn't prefer the aesthetic sound and imagery of the word magick better, I think the term Yoga is more accurate to what really goes on when someone does magick.  
In my book Become the Maelstrom I elaborate on this a lot more in the section – the Trinity of Self… but here is a basic summary quoted from my book below:  
True magick done right is union of the mind, body, and soul where - 
Soul, or spirit - This is the realm of pure ideas, ideas without limits, ideas without ideals. One idea is no better or worse than any other. It is the realm of language and symbols. It is here where every idea exists, and it exists simultaneously with its opposite, and every idea in between is also without. It is an empty void of possibility. It is a place of form without substance. A place of Perfect crystalline order, so vast and limitless that it is pure chaos. It is shape without content. It is the place where the voice of divinity echo’s silently for eternity.  
A chasm of paradox, where the abyss is finally seen as just an inverted hollow mountain. It is a place where every possibility and impossibility exist, but never with substance. Fire has no heat, though it will feel hot. Pain is without harm and can be a pleasure. There are no ideals, no one is better or more correct than any other. Everything is best and worst.  
It is where the butterfly exists before the mind applies it to the patterns in the ink of the Rorschach test.  
Mind - This is the act of decision, reason, intent, the experience of self-awareness, and interpretation. It is the seat of the ego, the realm of logic and causality. It is the part of the human experience that is most easily mistaken for the “Self” or the “seat of will.” However, it is best described as more of just an interface. A driver’s seat for the avatar of our biology. It creates the world around us by imposing meaning onto the subtle forms and substances of the totality of experience. IT takes ideas from the Spiritual aspect and chooses the ones that best fit as an overlay for the material world. Creating meaning, and giving substance to form, and form to substance. It is the eye and the mind that looks at a Rorschach ink blot and decides it is a butterfly.  
Body - The Material world. This is the realm of substance without form. It is here that the raw goo of causality waits to be molded and shaped by mind into form. It is the silent motionless dark sea of primordial Chaos. It is raw energy without direction. Logic without reason. It is perfect Order without organization. It is the great entropy at the end, and the stagnation of the perfect balance of the beginning before the bang. It is the Rorschach inkblot before we pick out shapes and give them meaning. 
In this way, A table does not exist as a table until is evoked out of the solid body of reality by a mind who decides it has the quality of a table, and that table idea is found in the spirit. Before that it is formless. It exists, but it has no meaning. you can still put a cup on it, but it is only a table when something decides it is a table. 
True reality is an emergent phenomenon of all three of these things working as one. 
You are an emergent property of fundamental consciousness that is coalesced and focused within the biology of life. Your awareness is focused on a brain, a brain that exists inside a universe, a universe that is an idea that comes from the soul, a universe that only exists within the consciousness of the mind that can perceive the idea of the universe as different from the soul. Each thing creates the other. Each is the other... Mutual arising.  
The Oroborus... the serpent eats its own tail and grows.  
Magick/Yoga/Elevated Consciousness is what we get when all three of these elementary extremes are blended and whole, without imbalance. 
To do magick effectively, you need to bring all three of these extremes into equal focus and make them all move as one toward the thing that you are consciously trying to do. You need to create a moment in reality when, even if only in a limited space for a brief moment, the thing you desire is real. The longer you can hold this, and the more intensely you can focus and make it real, the more power it will haven, and when you release all that intention as action, it will cause a ripple in causality which will cause the rest of reality to respond making that brief localized moment of reality REAL.  
This is why it is SO important to visualize and feel and think and know that what you are doing will really manifest.  
You unintentionally do magick all the time technically, but to do it intentionally, that is the trick, that is what the spells are for, and some would argue that all the spells are doing is aligning to you the true nature of your will, not actually making anything happen. 
But if all you are doing is saying a few words without feeling, without visualizing, without doing something physical, without uniting all 3 extremes... you are not doing magick intentionally... Your magick sucks and fails and sputters, and frustration follows. 
When all these 3 things are aligned, THEN there is no such thing as failed magick, magick will always work, it may not work exactly the way your ego predicts or desires, but it will work, every single time. 
At that point, it is easy to let go of your attachment to the results of your spells, or any other working. 
You will just know that you are empowered, and ascending, even if it doesn't always feel like it.