I think one simple reason (although would only be part of the story) is that he feasts on poor defenders. I don't think its a coincidence that he had great games facing Melo/Novak and then facing the Suns in general. But, when he faces an aggressive defender who takes away his right hand, suddenly he becomes much less effective.
I doubt that's the case. Of course, good defenders will always affect your play, but I think this has more to do with him than whomever is guarding him. In those poor games he still gets good looks, still gets to the basket, but for some reason he doesn't convert.
See, I disagree. I think there is a distinct difference in the shots he is getting in the games where he is successful and not, at least for the most part.
When he is playing a team that is successfully taking away his right hand (and thats the key defending him, do NOT let him get a step with his right hand), then he is taking a lot more unbalanced jumpers, and circus layups where he is just hoping for a foul. But, when he is able to get to his right hand, either off the dribble, or (my favorite play for him) a curl handoff play, going to his right, he is generally able to get close enough to the basket before he jumps that he actually has a chance to make the layups, and the rhythm pullups work more, because he is able to do it from his right hand, which he seems so much more comfortable with.
Basically, offensively, he is just a mirror image of Delonte West. If you take away his strong hand, he disappears, but if you give him that, he looks like a superstar.
I do agree that his aggressiveness on defense and the boards has been picking up though (along with the rest of the team.