Closet sliding doors - one sticks

I have heavy glass sliding doors in one bedroom closet. (They slide past each other on different rails.)

One of the doors has begun to stick, making it hard to push. I looked at the rail and saw black, sticky stuff. I'm conjecturing , faut de mieux, that it's rubber, somehow come off the gasket that (I assume) lines the underpart of the door that slides on the rail. Correct assumption?

I tried removing the gunk with several solvents -- alcohol and paint remover -- but no luck.

The doors are far too heavy for one person to remove just to examine the possibility of rubber (or whatever) coming off on the rail, and I don't want to call in reinforcements *if not necessary*.

However, if a gasket is deteriorating, I'd have to bite the bullet.

Thanks for any input.

TIA

Aspasia