"If your head is rocking when you push down on the cam lever perhaps you're trying to over-lock the hook into the vise. One thing I notice is if you don't put the hook into the jaws far enough or "deep" enough you might think you have to really crank down on the cam to lock the hook. You don't - you just have to get a little more of the jaw surface on the hook."

and if that fails maybe its a design flaw and its time to move on