Maybe, just maybe, he doesn't even try in training? Maybe he's as timid and doesn't assert himself, just like he doesn't in the games?
Ole, Ralph and now the manager that gave him a place in the Ajax team, all have refused to pick him. In case you've forgotten, it's within the managers best interests to win football games and they pick teams accordingly to achieve that. If he's not being picked, it's because the managers believe's he's counter productive to winning games, not that they hate him and don't want him to be a good player.
It's truly bizarre that he seems to get better the less he plays for us by some people on here, and it's even more bizarre how he himself has handled his big chance at a top club.