Thanks for the file, Andy.Man.
Please try the following. Select the Subdivision Surface Object called Lisa and copy it. Go to a new scene file and paste this. (I would replace the Subdivision Surface Object, it seems like an old one, more later about it)
Then subdivide the body in Sculpture and paint the mask, but: As long as the Subdivision Surface Object is active you will not see anything.
I would place anything but the “body” to the Subdivision Surface Object, the subdivision should be done in Sculpture—with the Subdivision Surface Object you take out the fine details which are possible with Sculpture.
I hope that helps.
Have a great weekend