I get the feeling that he's not trying hard enough. He has the skills, he just needs that grinta.
I don't think that he's not giving his all.
I'm starting to wonder if Marchisio isn't a player who doesn't have a huge amount of confidence in himself, for whatever reason.
I've seen plenty players who, on the face of it, have everything required to be a very good player, but they don't go and impose themself on the game and really take control the way one feels they should with the ability they have.
It is, of course, harder to impose yourself on the game when playing in a wide area, but Marchisio has also frequently failed to control the midfield the way he ought to be able to when played centrally.
Unless that changes, he will never be more than a solid players; a player who will look good in a good team, bad in a bad team and who simply lacks that extra something needed to be a real difference maker.