It is getting more and more difficult for young players that play the central role to make the break through in the PL. Many teams only play one up front and managers will not take the chance on a young player to lead the line.
What usually happens is that they get played in the wide positions which doesn't suit some players. If you look at Sturridge at Chelsea. He has made the break through but he doesn't look as good out wide but will Chelsea play him as the central striker. Likewise, Walcott at Arsenal.
For Delfouneso or Weimann, for that matter, to develop they need to play alongside somebody and not in a 4-5-1 formation. Above all, they need a run of games.