Some players never reach their potential. It all depends on how much first team experience they gather in the early part of their career, their grade of professionalism, the amount and lenght of injuries (throwing them back), and so on...
I wouldnt focus that much on watching the potential stat anyway - it takes away the fun
imo. Give your prospects the opportunity to play in your first team or even better loan them out to a club where they play as first team.
If they reach the age of 23 and you don't consider them good enough for your first team, get rid of them.