The players will only develop to their ceilings if they are a potential 100. Potential 9 players are good all around, and potential 8 guys are usually very good. Potential 7 players can be good with good ceilings.
For the other question, the little thing that holds a young player back can be his strength. If the player is weak, they tend not to do so good until they're stronger which can be developed by physic training