If it is at the bottom of the lock/handle side then the glass needs re-packing (toe & heeling) and/or hinges adjusting.
Mike at Agree-a-price frames, September 2010
It could be possible that the runner is damaged underneath, but they are adjustable, there should be a hole where you can insert a screwdriver at the corner, I had the same trouble so i took off the door and replaced the two runners
