Can't old players or slow trainers improve in quality through trainings or matches?

[cookizzz]

After further observation, I believe that skill progress is not reset daily, but reset occasionally or reset after triggering by an unknown event.

Today skill progresses of both my teams were reset. When I login upon matches completed, I notice that only starting players have skill progress remaining. The number of skill progress is exactly the same as those obtained through the match. I don't think all my players have zero skill progress before any match played today.