I've just picked up MudBox for the first time and tutorials are telling me to press and hold B or M to change brush size and strength, but moving the mouse while they're held down doesn't change anything.
Am I doing it wrong or is there something I need to press to fix it? I really want to use this function but it just won't work for me. I've tried tablet too but no luck.

This little white dot just moves around a little, that's all.