Just not very good
Pretty much this. He's obviously got a bit of talent, but he mainly looks good in the Championship because he is a good athlete and has a bit of pace.
I think his dad and his agent have somehow managed to screw with his head and made him think he should be playing for a top end Prem club, whilst in reality he's just another average England youth prospect who will never live up to the premature hype.
Same with Zaha, who looked great for his age group, but has struggled to mature as a player.