Color Balance not working on a mask

[GMP]

I'm doing a face swap and like the idea of the non-destructive method using layer masks. I have a layer mask on the face and just use the paint tool to erase the part of the face pic I don't want.
When I try to use Color Balance or Brightness-Contrast it only seems to work around the edges of the face and not the whole face. How can I get it to work on the whole face?