When your sliding patio door was new, it moved left and right along a level track.  But, now it sinks in the middle and on one end. The track is so low at one point that there is now an air gap at the top of the door.  And, every time the door moves across a low point it scrapes. This type of situation is not uncommon and could have been prevented with $4 worth of caulk.  Now, it will take hundreds of dollars to fix. The sinking of the