I checked with my DH, Leebob, and that's basically what he says too - throw on some more shellac and make sure it dries thoroughly, in a reasonably dry environment.

He's seen that mottling happen when he shellacs his bar tape in damp weather, but slapping on another coat in drier conditions takes care of it.

As for the clear shellac that Lisa suggests, I suppose that's assuming that you're happy with the current color and would rather not make it deeper, I would guess clear shellac wouldn't change the current color much. You could always use the amber shellac again and make it a much richer color.