probably you just get closer to his maximum potential.
progression slows down naturally when getting closer to max potential...

Same for me, with 2 players. Their skill is set on 2*, with potential to be somewhere between 3,5 and 5*.
Both are 15 y.o, with standard development rate.
Not happening on a very similar skill player who is 18y.o, with less potential. Werid.

Player stars for actual quality and potential is one of the most inaccurate things in this game. That is also valid for scouts. Especially for young players, so you shouldn't rely much on them.
As for the speed of training, it's lower to train skill of high value than skill of low value. I think It's roughly half of speed for every 4 skill points, so if you have some skill on level 10, your player will progress 2 times faster than he will progress once that skill reaches level 14.

Also keep in mind that when player reached his potential, his progress will suddenly almost stop. It happens exactly at the moment when he reached potential, so you should see big drop of progress immediately.