Some weapon and shield plaques may have activators that do not function when you first enter the cell - this is common if you teleport in (or use the coc console command).

The easiest fix is to make a save (not a quicksave) and then reload it.  You can also zone out of the cell and then go back in to reset the activators.

This is a know game bug:   https://www.google.com/search?q=skyrim+weapon+shield+plaque+glitch