First, that's an interesting overall video about double hung vinyl windows.
I saved it for future use since I participate in a couple of real estate
investor association online discussion groups and people often post
questions about fixing window problems.
I don't know the answer to your question, but it does seem like the pivot
shoes are getting stuck in the track in just one area. Since it is on both
sides, it makes me wonder if it is somehow supposed to be that way, or if
there is a common problem on both sides that is compressing the track near
the bottom of the window -- like a window sill pressing on and compressing
the track on both sides(?).
I happen to live near a window and door parts place called Swisco in Camden,
NJ, so when I get stuck I sometimes just go there in person and ask them.
They also have a good website where people can diagnose problems, ask
questions, find parts to order online, and watch instructional videos.
I don't know if any of this will help, but here are some of their website
links to their home page, a video about the pivot shoe, and a list of their
videos, and a link to their discussion group where people post questions:
http://www.swisco.com/
http://www.swisco.com/videos/view34
http://www.swisco.com/videos/list1
http://www.swisco.com/discussions
Good luck. If you figure out what the problem is, let us know.