Fixing our front door

Our building has communal areas, our flat's front door is within. Recently, the locks stopped working well, as if the door suddenly didn't quite fit any more. I peered at the door in some puzzlement, opening and closing it, trying to understand what had changed, if a hinge was failing or whatever. It was R. who figured it out and fixed it: a small stone had somehow lodged under the thin piece of wood running along the floor under the door, pushing that wood up a little.