As athletic as Robert Williams is vertically, his lateral quickness is a weakness. When he gets switched onto a guard, he doesn't move his feet particularly well.
For my mind half of it seems to be his stance. He stays really vertical - maybe in anticipation of a step-back?
He does have a very upright stance. Could be a large part of the problem.
I think he's always trying to keep his feet under him to go for the block attempt. It's an experience/discipline thing... as he gets older, he'll (hopefully) learn that going for the shot block isn't always the play.