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well it could be that the spine there is uneven and needs to be fixed, I can't tell for sure from the pics.
I think the magic marker will tell you which stroke works then stick with it until you fix...

have you seen the bevels under magnification? Are they nice and flat or there is a double bevel in that area?

+1 on the magic marker, try different strokes and see which one works best.
Also as nun2sharp said there might be some frown left but the magic marker will help you find out what the issue is.