I think experienced players should have encountered this problem, especially those who do power training.
I wish to understand the main cause of the problem

The same 18/19 yr old player that I have, initially was a fast learner when I bought in via transfer market.
So I decided to field him in every match possible to train him up. Feed him with just one or 2 green booster everyday to pump up his stat (not to the extent of power training yet as I'm not a token player).

Then the strange thing happened. During mid-season, his training speed and stat gain started to reduce; in quite a significant level as I can notice it with ease. That's very frustrating as your faster learner has suddenly lost his talent not that he had grown older.

I understand that during 18-21yr is the fastest learning stage where player loses training speed as he grows older, but why my player loses training speed during the same season???

Anyone with same experience?