Shellac mystery spots

I'm finishing that stairsaw I mentioned a few days ago. I'm rubbing 2# shellac. There are two spots on the wood that just won't take the finish. I'm also having trouble getting the arrises to take finish, but I think that's just my technique.
I filled the Ash with rubbed pumice and 1# shellac: nice smooth surface.
One of the patches is light wood, the other is dark . I tried rinsing and wiping with (separately!) Ronsonol, acetone, and mineral spirits. All the other surfaces are coming along fine.
Any ideas what's going on?
"Keep your ass behind you."

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