I googled a pic and it appears the binding post plate screws may have a torx or star shaped screw. Just find the right bit, remove the plate, and you should find a loose nut on that post. Sometimes there is a small projection of the plastic around the metal post that prevents it form spinning. If that broke you could get a replacement post or just attempt to glue in in place. Should be an easy fix.
While you are in there tighten them all down.