It is either what Steve suggested, a link off, or what I encountered on my 108. The connecting rod between the inside door handle and the lock assembly had somehow gotten bent. When locking the door, the button would go down, but not far enough. The bend in the linkage short stroked the throw, which kept the lock assembly from completing it's travel. Result - the door looked locked but wasn't. Check the 90 degree bends at each end, as well as overall shape of the long linkage. The 90 degree at the lock assembly was the culprit on mine.
73 280 SEL 4.5