I had same problem. I had to replace the rollers. Not too difficult, but door very heavy. My particular model you have to remove the lower frame to get to rollers. Once you get the lower frame off you can get the roller’s assemble out to disassemble and replace roller from the bracket.

There are three sizes of rollers. Somebody before me installed too small rollers and the adjustment was maxed out and wouldn’t lift the frame off the trace causing it to drag. How do I know this cause I brought the exact size of the old ones and then door still was hard to close. I got bigger wheels So I could raise the door off the track and have some adjustment.

After I cleaned up the trace and got the new rollers in it slide smooth as glass. You have to be careful it you do this yourself, the door heavy, remove the lower frame off the glass using block of wood and dead blow hammer. Going back together wasn’t too hard other than maneuvering the heavy door back on traces without pinching fingers or smashing toe.

Where to purchase one of those caps??? Thanks in advance