I meant to reply to this the other day but didn't get time.In many ways we have had the big problem Eng;and have had, which is producing great players to 16/17/18/19 but ultimately them not kicking on. There is a variety of reasons for this. Last season we played more lads 21 and younger than any other team. We played more than treble all but one team. We always have done really, but they don't kick on. Thats the question that needs to be answered.
In my opinion, the simple answer is that junior football and senior football are very different beasts. It's not necessarily possible to ensure that players who were good at 16 are still good at 20, because there are natural plateaus. Despite what memes and life coaches might want you to believe, you can't just keep getting better and better at something forever, you'll eventually be as good as you're ever going to be. It's not limited to football, the same applies to everyday jobs. You'll often see someone progress really quickly through a company, but then they naturally reach a point where they stay, because they don't quite have what it takes to move to the next level.
I think fans often fall into the trap of assuming that a player's progression will continue indefinitely. Often people seem to think that just playing more games will automatically make them better, which is a really odd viewpoint in my opinion. I think of players like Rodwell and Galloway for example, and would say that they were never particularly good. They were adequate, but no more than that. People look at it and think 'well if he can hold his own in the PL at 16 just imagine how good he'll be at 23', and then when they're still only as good as they were they believe they've gone backwards. It's like when someone videos a parachute jump, and you think that when the parachute is released the jumper has gone back up in the air, when actually they've just slowed. It's our perspective that's wrong, not the descent of the jumper.