Why do players stop improving in skills? I'll recruit players who show high growth in 5-7 areas and by the time their a junior, I'm lucky if they have 1 area left? Am I doing something wrong or is that just part of the system?    

That's the system....

It's a byproduct of the Potential system. Back when I first played this game, there was no potential system and players continued to progress and the higher  their rating in a particular category, the more minutes it would take to make them improve.

Personally I liked it better as it enabled people to build and coach there teams the way they wanted. If you wanted a Big man who could shoot, you could put perimeter minutes into him, but thats been gone for a long time now.

It also gave people that don't always recruit very well an opportunity to still build good teams. Problem is a lot of teams start too look too similar at times and with the focus on the beginning ratings (since everyone moved at roughly the same pace give or take for WE I believe) the coaches who did recruit well or had better prestige often got the better players which also means somewhat of a head start on a good team.

I would personally have no interest whatsioever in play-dough players.