Well we have had the same problem here in Iceland, now we have 65 active users, but I'm not sure how many are actually active, usually we have had around 40-55, it is hard to make a good u21 team because you do not only have less active users to train your players but you also get less drafted players and even less of good prospects from that draft.
I'm not going to say that we have had a great u21team (maybe great if you look at the country size), but we have had a pretty good success for the last seasons. That is simply thanks to our u21 and NT managers, they have watched all the inactive users and if there are any good prospects there that are going to be released on transfer list. They have made sure that the players that are sold to teams outside our country will be trained properly, or at least do their best to do so.