Did you let them play competitive match before you start to powertrain them? At maximum motivation even a fast trainer can train very slowly, you have to give him enough pitch time so that you can train efficiently.

"A player with pitch time train faster" is one of the few important key mechanic mentioned in the official help section. What it didn't tell you is that a player with no pitch time train extremely slowly, so slow you can mistake a fast trainer with an ordinary.

I was able to keep a 22y scout 7 stars for 3 seasons with a morderate green pack budget.

And if you let a player with max motivation play, his form will likely be a 8 or 9.
I guess you sold them too soon and never let them play, before you could realize how good these players are.