Post a screen shot of your player's skills and maybe we can see what the issue is. For example, I claimed a 23 y/o DC/DMC at 106% Ability, 36% Quality on Sunday (10 Jun) and he is now at 108%/37%.
Sunday ----------------> Friday
Ability 106% ----------------> 108%
Quality 36% ----------------> 37%
Defense 118% ----------------> 121%
Attack 98% ----------------> 99%
Physical 103% ----------------> 105%
So he is training and developing, and I have not been powertraining him or using lots of greens (just daily training with the rest of the team and gains from playing).
If the player you selected is older, he just may not be a good trainer (like other older players you already have).
One of the things I do is take a screen shot of a player when I first get him and compare his stats over time, that way you know for sure if he's developing and can see the actual numbers change.