Mate you do not have to register any players who are 21 or younger on 31.08 of each year either in any UK competition or in champions league.
I play with Liverpool 3 years now and I have not yet registered shelvey and kadlec and they can play, but without number. Although that's strange, since in real life every player must have a unique number, however if there is no problem with the game, there is no problem of course with me.
Have fun and good luck to Peterborough.
"i have a few promising youngster aged 16-19 but it says they're unregistered"
For some reason his under 21 players are unregistered, bandura that is the problem although you did give a good summary of the registration rules :).
I think it has something to do with nationality but I'm not sure. I know your only allowed 3 non-eu national players to be registerd but I not sure if the same rule applies to the under-21 players. Could you post which players you have the problem with