Mine get stuck on the drivers side coming down and going back up, about a couple inches from the top. I've found that if I pull the top of the glass in towards me it goes like it should, like butter. I guess that's not so much an answer of your question which was what's causing it as a way to half-done fix it. But what it seems like to me is that while doing that helps get the window past that sticky spot without any problems, if I hold it like that the whole way, the entire travel of the glass feels much smoother. I've always just attributed it to the geometry of the glass, possibly the geo. of the glass versus the geo. of the door and/or window itself. I dunno. Again, not really an answer. I guess this was kind of a useless post haha.