I think the issue is that people want him to do well. They expect him to do well given his apparent qualities. But he doesn't seem to get into the team enough or get put on when it seems sensible. And when he does play, he often end up giving the ball away and looking off pace. To be honest, I think he's being mismanaged along with the rest of the side.
That's not to say Pochettino can't sort it out in time ... but we're trying to throw in all our attacking players at once instead of building a sensible looking attack.