I once flattened a piece to use as a backing, and then I torched the pith side to correct a crook. Not the outside, you understand, which would be kinda dumb for a backing. Anyway, the glue side was flat from sanding, but then bulged as it was heated. The back flattened and the flat glue side bulged out slightly in the middle. So, I did the whole thing and re-sanded the glue side flat.
So, it flattened it flatter than it was, but not all the way flat, and I still used it as a backing, no problem. But not flat, flat.
Is your boo just too small in diameter?