I think he is being over managed. When he first came he was raw and unpredictable, had a bag of tricks and always managed to get a shot off.
Now he gets the ball and he has five conversations going on in his head. He has become risk averse. You saw how long Pep was coaching Grealish before he came on. Literally pecking his head for 5 minutes. The result of this is a player that doesn't use their own instinct anymore, they always play it safe.
To be fair, teams are much quicker to double up on him, and he regularly gets three players marking him. Anyone who tries one to one can't cope - as we saw yesterday, when he forced one yellow, and should have forced two, within minutes of coming on.
He's arguably the best dribbler in the world, but nobody can beat three players when they're happily sitting with the other seven outfield players in a low block behind them, so it starts to look like the old 'Pep had ruined him' trope.