I have one of the red and grey Tipton ultimate gun vises. Recently I set a rifle up in it to mount a scope and had a totally inability to adjust the elevations on the clamps holding the rifle. When I try to tighten the red rubberized knobs to the correct elevation they seem not to grip and twist right back out so there’s no tension to hold them in place. Has anyone else had this issue? It totally could be a user error but I’d love to get it working right again.