Ten years ago, we bought three Kohler toilets.  One of the seats recently started to slam shut.  I purchased a new hinge and installed it.  Within a week or so, one of the pins would slip out of place for no reason.  When I tried to replace the pin, it seemed as if part of the pin had turned so it could not be replaced.  I called Kohler and they sent out a new hinge.  Same thing with the second one.  I figured out that I could replace the pin if I move the hinge so that the hole for the is in alignment.  This occurs when the seat is fully up.  I don't understand why this is happening with this one seat and not the others.