mine need only a very little tension.
I would use 1200 grit and sand the slipper pads slightly.
Clean everything with brake cleaner.
My guess would be the diff. The slipper spring puts some massive tension on the pads when fully tightend, I would never ever tighten it even half ways. And even if it is glazed and oily it should not slip much. Some slipper use teflon/ POM slipper pads and they have less friction without oil on it so my tip is the diff. As chimp said compress the spring first!
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