Something is hanging up, which means it will stop so that you can re-sync. When it stops, let go, then hit the button again, the other side should come into square, it will stop again, then start back in square (until it is unsquare again and stop). That is what is happening...now you need to figure out what is causing it.
Check that the rubber seals on the outside are not getting caught in the roller below the track. Sure sign is a black roller wheel. (should be white)
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