It may be that the mirror has slipped down a bit on the body that you are having problems with, compare it with your other FE bodies and you should be able to tell if this has happened. The glue that holds the mirror into place may have softened at one point. If this is the case, you may be able to soften the glue enough by gentle application of heat from a blow dryer to allow you to push the mirror back to its original position, and the glue will reset as it cools.