I almost bet your problem is this part called window guide roller. here is some pictures. Yours should be almost the same if not exactly, mine is from a 69 convertible.
This is a pictures showing the broken nylon roller in the center.
This is a picture of what it looks like attached to your quarter window.
Close up of broken roller
The roller was held in by a special rivet so I had to have one made at a friends machine shop.
Here is a pictures of the drilled out rivet on the right and the new made rivet on the left side
Here it is finished and restored below
If you want to see all the pictures go here
Click HEre