Soft top. Difficulty sliding up plastic windows.

I recently picked up a soft top and find it easier to fold and unfold it than i expected - except for the windows. One slides up relatively smoothly but the other requires me to do so much pulling that I've started getting a sore back. I wonder if the channel it fits into is just too narrow but I can't see any way of adjusting it. I've also tried liberal amounts of WD 40.

Has anyone else experienced this problem and does anyone have any good tips?