Probably cam wore out - dress up the cam with a file and use a brass washer for the contact point on the vise.
You see that a lot with the AA knockoffs. You don't see that with Thompson AA style (patented in '52) or Herter's AA style (still trying to find out if there is a patent). Veniard (English manufacturer) makes a good AA also.
Don't understand why novices don't use these quality American made vises for entry level tying. And it's recycling.