It's more usual to click on Select Subject before Select & Mask. You need to get the selection close to what you want, and as a last step use Select & Mask to clean it up.
What View setting are you using with Select & Mask? Try setting it to Onion skin and Transparecy to 100%

Also try turning off other layers. You can see above that I have nothing behind the selection and can see what I am doing
But if I have the situation below with an underlying layer filled with black

Then Select & Mask is difficult to use because the black layer below is visible.
Note I have change the View setting to Marching Ants so we can see the selected area.

Does this help at all? If not, we need a screen shot as requested by c.p.