Actually, just also thought that if any washing soap is left to dry on the rubber seal this would definitely increase the friction.
Remember unlike standard windows that have a nice loose frame, frame-less windows require a tighter seal to stop wind noise and rain. Also check the door alignment, as this will also effect the window sealing against the rubber and bounce the glass. As if the door isnt closing enough it will be hitting the edge of the roof window slot rather than engaging correctly and creating excessive friction.
Just a thought... Very easy/cheep to try :wink: