One possible cause and seen a few times by me, is that someone has filed down the foresight, thus bringing up the POI with backsight at lowest setting. Have a look at the foresight for signs of tinkering and of course ensure that the backsight is indeed at its lowest setting. If foresight has been filed down you could try making a brass bead of some sort and superglue on as a temp measure to see if this cures the problem. New foresight possibly?
Mike95