Level 15 training

I've been on level 15 for two seasons and have noticed that the skill points from training are much harder to get. is this normal? I used to regularly get 5 or 6 from a cardio workout but now it's more usual to get two. is this something to do with level 15 or has something gone wrong somewhere?

It is not your level, but your players are likely older and have gained a star.

Thanks that's true