There are two aspects on this.
First of all, the development of a player's ability in the game can be compared to filling a container, whereas you only will be informed when the container is full, ie when a skill point is obtained in the old version or the player gains one per cent in the new version. This is very noticeable in the new version when you practice players they gain percentages in circles pretty often. All practices has affect, but we can only see in this case when the player gains a full per cent.
It is also said that training players contains a random factor, i.e. that there is bit of a luck involved etc. I think the effect in the end of any such random factor is very small, but it is perfectly possible that it exist.
Secondly, some players develop a lot faster than others. You can spot these players by looking at their value in relation to salary and maybe also ability. If an 18 y/o is extremely expensive in relation to how good he is -- it is because he has a lot of potential.
I cover these questions in a guide I have created for new players, if you want to check it out you can find it at:
http://forum.topeleven.com/tutorials...s-players.html