There have been many post on this topic but good luck searching for them.

Those tracks were pinched at several locations and those piches have to brought back out. After spending a lot of time and wasting a lot of good swear words, I cut off a small length of the rubber and guided it through the channel/track. At any place it got stuck, I opened the track a little.

You can also use a little lubrication but stay away from silicone containing compounds and petroleum-based compounds (like Vaseline). Use a water-soluble lubricant (personal lubricant, available in any drug store - get the cheaper store-brand).

The Factory Assembly Manual has a drawing of the tracks/channels and the locations of the pinches (as I dimly recall).