Make sure your jaws are fully threaded onto the drawbar. The further out the jaws sit on the drawbar, the further back your adjustment knob will need to travel rearwards on the collet threads to contact the cam lever. Disassembling the vise to visually verify that the jaws are able to fully thread onto the drawbar might be a good idea. For that matter, an occasional full cleaning, lubing and reassembly isn't a bad idea.
I think that must be it. I can't image this happening in any other situation on my HMH standard. When changing jaws I always fully loosen the adjustment knob and screw that jaws in all the way, then back off just enough to level them out. That ought to do the trick.