Didn't Clark play A LOT of minutes at the 4 last year? That is my theory as to why he did so well last year. He was usually matched up against a post, but played on the perimeter. This year, he can't shake loose from the quicker players that are guarding him.
That, and it looks to me like he changed his shooting form (for the worse, actually). And at this point, he is fighting the mental side of things. He probably won't shake free this season, but hopefully in the offseason he can get away from the game for a few weeks, then get a fresh start.
I've been saying since he signed though that he probably wouldn't live up to his ranking. The kid was RAW in the skills department coming out of school. What he did in HS was play on a team, that had a system, that was kind to Cam. Kind of the up and down, run and gun. In a system like that, Cam's athleticism can help him overcome his relative lack of skills. In the halfcourt game, not so much.