I spent two decades as a driving instructor. I had pupils who passed after about 10 or 12 lessons and others who took over 100. Look at it this way, if all of us had to achieve an A level in maths when we'd never even added two plus two before, how long would it take? Some would sail through it, most would eventually get there after various lengths of struggle and some wouldn't get there while they had a hole in their ar$e.
Forty lessons is now reckoned to be the norm, the standard required to pass is way higher than it was twenty years ago. The overall pass rate was around 43% last time I checked, it has never been over fifty percent in my memory.