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 track is typically due to the deterioration of the underlying wood foundation support. You have "wood rot".
Although the edge of the patio door track on the outside may be atop a cement ledge, the majority of the long track is supported below by wood.
If this wood is soaked with water every time it rains, it will soon deteriorate and become so soft that it will no longer be able to support the weight of the door as it rides along the track.
To keep rain water from reaching this supporting wood, a bead of caulk is run from one end of the track to the other along where the edge of the track meets the cement.
Since shifting over the years and harsh weather can cause the caulk to crack and lose it's seal, it needs periodically to be removed and replaced. While your at it, caulk the joint where the track meets the vertical jamb at both ends. Sometimes, settling can create gaps that will allow the permeation of rain water.
A sinking track can be level once again in a matter of hours.
The track can be unscrewed and separated at both ends to expose the rotting wood. Then, the wood and track replaced. This typically requires a skilled individual.
If the sinking has not yet become severe, composite shims can be used as the track is merely pried up a each depressed location. Then the caulking must be addressed.
(Also read the posts about Pella Wood Rot and Weep Holes.)