I dunno really. As an aspiring leg-spinner in my youth, I can certainly attest to the fact that not every ball turns, but again that is a question of skill too. But even the very best spinners will bowl many balls that just do not turn. It's a collision of many different elements that just fall into place to let you have your day with the ball.
I have a number of theories, which would allign with real life too. You have to find that area of the pitch that is roughed up enough to influence turn, so trial and error and using the angle of delivery helps. Of course what does not help is your captain putting you in after 5 overs when the ball is still new and the pitch is still relatively fresh. The ball will just skid off the surface.
Another thing is in-game timing. You need to turn the left stick in the direction of your main delivey's turn 3 full timesto max out the spin meter. Time it too early, and the meter starts to drop. Do it too late, and you dont get the 3 full turns in. Really as a fresh career player, you need consider about 1400rpm or so to provide the potential for real turn.
These are just my observations though. I could be completely wrong however, but I do get balls that turn a good 30 degrees. Its very intensive. Unlike pace, where your main focus is the jump timing, with spin your main focus is trying to get the ball to behave as you want 100% for each and every ball bowled.