Some faces refuse to be painted...

Hello folks...

I'm using a black mask for a material that I want to reveal over another. With the mask selected, I'm using the paint brush to paint in the areas I want to be that particular material. I hope that made sense. 

Now, there are some faces, small and at right angles to the rest, that refuse to be painted / erased. They remain the color of the material beneath it in the layer stack. I've tried painting by face, but that had no effect.

Thanks.

Windows 10

Painter v7.2.1