Any pictures?
is something not sliding into place, forcing the metal chrome piece to bulge- open up larger than the footprint of the gasket it mates with against the car?
im assuming, based on the picture ( below) you posted in your led thread, you are trying to slide the chrome bit back over the two verticals ribs, and it’s the black gasket against the car that is not complying - you are referring to in your Q,
could that loose rubber piece along the right vertical rib piece be causing “the bulge” when you slide the chrome piece over it?