Substance Painter - Transparent Layers on Masks?

Hello all, not really sure how to describe this as I've just began using Substance Painter about a week ago now. I've noticed that when I to paint using a mask, my paint is never 100% opaque, and it often leads to colors stacking on top of one another.

Painting on top of one another results in this overlap, despite the opacity of my brush, the flow, all being at 100%. How do I make it so it's the same color all the way through?

Likewise, if I try to paint on a lower below (the red layer is below the yellow one, for instance) the red is still visible underneath and affects the yellow color rather than being fully covered.

I'm going nuts trying to solve this!