One of the big questions I felt I needed to answer was: why is magic so unreliable? No supposed explanation would be complete without addressing this issue.
This depends on what you mean by "reliable." A reliable car starts every time and never breaks down. A reliable employee is always punctual and has 100% attendance. However, that is not the only way to be reliable.
Imagine a drug that improves survival rates from a given disease. Without the drug, 10% of sufferers survive. With the drug, 25% of sufferers survive. Most people would call that an effective drug. They would accept that it did not work every time — probably due to a variety of unquantifiable variables — but they would not use the word "unreliable" to describe it. Indeed, if competing drugs produced a 20% survival rate, the one that produced 25% may be called "the most reliable".
Ditto with an eminent surgeon whose patients survived 90% of the time, when other surgeons at the same hospital treating similar patients achieved only 85% survival rates. He is a reliable surgeon, but some of his patients die.
So now imagine a form of magic that
demonstrably and measurably improves the probability of a favourable outcome but without
guaranteeing a favourable outcome.
If, for the sake of argument, it were possible to do control studies with measurable results and you
consistently demonstrated that there was a favourable outcome 65% of the time with magic, and only 50% of the time without magic, that would be evidence supporting the hypothesis that magic is efficacious (capable of being effective), without it being effective every single time.
There are various popular theories and models for how magic might work.
If it relies on the intervention of a paranormal entity (demon, angel, spirit of the deceased) then the entity may be capricious and choose not to intervene, or may have a conflict of interest if another practitioner is invoking it to achieve the opposite effect.
if it relies on psychological or psychosomatic factors (placebo, nocebo, fear of the curse, suggestion, etc.) then the effectiveness may vary with the intelligence, education, mental health, and cultural background of the subject.
If it relies on some sort of blast of psi energy from the practitioner, they may be fallible. Alternatively, the subject (victim?) may be particularly resistant or resilient. Even Mike Tyson landed blows that did not knock out his opponents.
If it relies on some sort of release of power from a concoction of herbs and entrails, there may be some variation in the quality of the ingredients, or in the way that it is prepared, stored, or administered.
If it relies on some sort of twisting or selecting of consensus reality — choosing one reality out of the countless possible ones — then the combined effect of all other consciousnesses may be strong enough to prevent the change or selection.
And so on. In every field of human endeavour, nothing is 100% reliable. The best lawyer still loses some cases, the best boxer may be floored by a lucky punch, a top cyclist may hit a discarded water bottle and crash, a fill of bad fuel may block the carburettor of an otherwise reliable engine. Why should magic be any different?
I am not a believer in magic, but I am just following the argument where it leads.