You are right it is not a spanwise issue, it is the hub. It is what I was trying, perhaps rather un-eloquently to describe earlier. You need to sand the side of the hub, not the front or rear faces of it. If it will not stand vertical then the side of the hub that is falling to the underside is the heavy side so sand this away. It works best with a block sander - sand it flat until it does not rotate any more.

James