They just aren't that good is the obvious explanation, compare them to 'proper' young footballers in the AM team and they don't measure up. It's not just a problem Man U suffer from, its typical of Sky and the English media as a whole...hype up average players and teams to be world beaters and act all surprised when they don't perform.
Well its AB not AM for starters. And Cleverley has been injured most of the season and started about 10 matches for United in his career. Pogba hasn't even started a single league match yet. The Bilbao youth players have been playing first team football a lot longer so are much further on in their development.
I mean look at Munian and Pogba:
Both 19 years old, Munian has played 117 games for Bilbao, Pogba has played 6 for United.
This is why I keep saying we should have tried Pogba more this season to help him develop. Even Cleverley has started quite late for us (that may be more due to him being quite small though)