A contractor from Burlington says:
September 29, 2015
One of the internal gears must be missing or have a broken tooth. Because it works most of the time but right at the end, where it pulls the window in for a tight seal, it doesnt grip.
Tom from SWISCO responded:
October 1, 2015
Take a look at our 39-239. I believe this is a match. You could likely re-use your black shoes.