Double check to see that the spring back is keeping the holder in contact with the back. Also check where the bellows is connected to the rear standard. I don't recall what camera you have but if the bellows is held on the standard with a retaining plate and screws, they may be worked loose, or you may have some sort of seperation of material at the back of the bellows on the side of the over exposure.

Another possibility is the holder frame could have some build up of gunk in the corners or foreign matter imbedded, especially if it is wooden. So check rear frame for dirt.

Just to make sure on the holders, use a straight edge and check for warpage. A slightly bowed holder could allow a light leak where it seats to the frame.

I suggest these only because I have run across all of them at one time or another on various cameras of mine or those of friends.