A quick learner from Morgantown WV says:
October 9, 2016
I have a pivot shoe that seems to not slide well at all. As a matter of fact, after taking my window out and noticing the two shoes were at different heights, I found it very difficult to move the shoe. Do they get stuck sometimes, and why. Is it the shoe or the frame causing the problem?
Paul from SWISCO responded:
October 11, 2016
If your pivot shoes aren't broken in any visible way, then give the vinyl track a spray of silicone and see if that helps. If not, they may be damaged and will have to be replaced.