For whatever reason, Al&Sharon's link won't open.
Sight down the track to ensure it hasn't been dinged or warped, impeding the slide. If so, be judicious with the tool of your choice, and tweak it.
As part of our almost continual mod-remod activities, we have removed several interior window extrusions and completely removed the large window behind the navigator chair. They seem to be relatively idiot proof as far as racking out of square during installation, but hey, all things are possible!
If everything appears as it should, I would vacuum the track well, take a stiff toothbrush and scrub the track and sliding portion of the glass. Vacuum well again, then use the toothbrush and soapy water and scrub the track yet again. Allow to dry, then apply silicone spray liberally to the track and work the glass back and forth. Should do it.
Fair Winds and Following Seas
Terry & Rosalina
2007 Winnebago Journey 34SE
Retired Navy Mustang & Navy Wife