That's most definitely due to lack of playing time for the youngsters and a huge pressure when they're trying to live up to expectations a the slightest chance they get, The most recent case would be Yerry Mina he came into with a big potential as a CB but had a very limited playing time due to Pique and Umtiti's partnership at the heart of defence, but whenever he gets a chance the pressure is always too much.
You are right about that and even at the front and midfield the likes of Aleix Vidal, Paulinho, Paco Alcacer alongside many others failed to make an impact before they were released, and we have some more who could be following the same path.
Last decent signings were made in the 14/15 season where Suarez, Rakitic and Marc-André ter Stegen came in but after that signed players haven't really shown their potential thou Barcelona is still very successful in La Liga thanks to Messi.
If you remove Messi out of the picture, Barcelona would face a lot of crisis just like Real Madrid Just had a taste of their own.