I've used the 'mask' effect a few times. But suddenly it no longer works. Even if I create a brand new scene, and just draw something on a layer, using 'b'. Create a new layer, draw a circle, with 'b', then fill it with 'k', when I set this second layer to mask the first, nothing happens.
Weirdly, if I 'Preview' the scene, the mask is working ok...