When I first encountered this problem a couple of years ago, all that
anyone previously had done was lube and clean the tracks. I sprayed the
roller axles (probably used WD-40) and it made a dramatic improvement.
The problem has now returned. Spraying the axles no longer has much effect.

What's really puzzling is that the owner says even during the winter
(central NJ), some days it will be fairly easy to slide; other days not so.

One side of the door faces the normal living space, so is exposed to
ambients around 70F. The "outside" faces an enclosed sun room with a
jacuzzi. During the summer it gets very hot; I don't know how cold it
gets during the winter.

The wheel axles face the outside and are easy to spray without removing
any trim plugs.

The door is a bit bigger than 44"W x 88"H. It's double-paned, so even
under ideal conditions its heavy weight makes it somewhat hard to slide.
As I recall, its a metal frame. I don't know the brand.

I don't feel any flat spots on the wheels or the track, but I haven't
been in the house for over six months; things may have changed.

Thanks for your suggestions.

R1