The Axis is a bit weird in the back with that block to deal with.

When fitting it should go in straight down, not tilted. Both bolts should engage easily.

There should be no sanding past where the barrel nut fits. Only sand forward of that point. Do what Robin said with the paper. Slide it between barrel and stock to see where it 'sticks'. Sand that area some more.

Be sure to use a dowel to wrap the sandpaper around. I'd use a 1" dia just to leave the channel a bit big.