It's not the instructor's fault, per se. Sometimes the kid is so good that the teacher gets jealous and holds the kid back because the instructor doesn't want the kid to grow up to be better than him.
I've actually heard that one once. Sadly, I wish I could have made that delusional excuse up.
I was being sarcastic and quoting from the parent's point of view because it will never be their child that can't do it. Nine times out of ten though a child that is actually that good will be given more training not less because to have a student grow so much better is a point of pride.