I think you’re referring to the little bar that covers the opening. Mine was loose for a long time, and finally fell off when I wasn’t looking. It doesn’t appear to effect listening, but I’m worried something will get into the openings.
I had a lot of trouble with the pre-cut adhesive for the back cover. When I tried to peel the lining off of one side, it came off of both sides and tangled. Is there a trick to applying it?