One of the main reasons that our young players don't take the England under 21's serious is because the England set up its self doesn't take them seriously. There is no natural progression to the full England team. Example:
Kyle Walker has been the right full back for the England under 21's for the last 3 or 4 tournaments - hardly missed a game and performed well. He also a first team regular at club level.
England's first team had an injury to the regular right full back. What should of happened is the Kyle Walker should have been an automatic call up to fill the position. He wasn't. Hodgson played Phil Jones at right full back instead! What is Kyle Walker supposed to think about that? He probably thinks he is wasting his time with the under 21's and he is 100% correct. If you had automatic progression from the under 21's to the full side in cases of injury or suspension then the younger players would know that their chance will come sooner or later. Hence they maybe a little bit more interested in turning out for the under 21's and putting in a decent performance or two.
If you regularly turn out for overseas tournaments, play the games, be in the under 21's squad then immediately get overlooked when your position in the full side becomes available we shouldn't be to surprised that a lot of the players can't be arsed in the first place.