Hello Warrior,
it's difficult to give you a piece of serious advice here. By saying this :
"If I offer a training program (potential allstar to superstar), many owners drop out and some don't even train the players at all. It's frustrating to discover all this. Our scouts also get little feedback, which makes it difficult to help where needed."
you made a summary of all what national coach is about : frustration.
Coaches in the game have a wide array of approach to the game : casual, regular, invested and hardcore. Of course you get the best training by getting harcore coaches on board BUT those are much more likely to train NT players, MVP+ guys, and are therefore of little use to you.
So you have to rely on regular and invested coaches to get your U21 trained properly. A solution that might be worth considering, id relying on hardcore coaches with starter teams e.g. fresh Utopia teams or newcomers that are doing very good despite a rather fresh experience.
I did that with my second team and it produced Hemeryck and Ballon, both players that were a staple of the U21 team when they were in it. I achieved this with players potential 6 or 7, and a coach level 4, do it's definitely doable by anyone.
Putting the emphasis on the financial gain is also good, because as you said, the boost is huge, whether if the player is sold afterwards or kept in the team.