Can't Get "Select and Mask" to Actually Mask Anything?

I use photoshop to clean up my traditional artwork for printing. Mainly I use "select and mask" to mask out blank backgrounds so they'll come out pure white on paper. I used to just open photoshop with a white CMYK background, place my scan into Photoshop, and then use "select and mask" to hand mask out my artwork, and then click "OK", and my artwork is nice and pretty on top of the white layer. Now when I use "select and mask", all it does is create a selection and nothing is masked? What am I missing here? Someone guide me, please! I'm losing my mind!