Brush tool not working

I'm sure Rob and Bojan have nailed it, but a couple of tricks that can help.

Shift click the layer mask to temporarily disable it. 

Alt click the mask to make the mask fill the document window.  This is a super handy trick for revealing fine detail.

If you are dealing with an Adjustment layer, do something radical to make sure the adjustment is too subtle to show.  You can step back with Ctrl Z to undo once you have learned what you needed.

Ctrl click the mask to load it as a selection, and hit 'Q' to go into Quickmask.  If the mask has areas darker than 50% grey you would not see Marching Ants, but Quickmask would reveal any low opacity brush strokes.

Resetting the brush is a really good step as it restores default blend mode and 100% opacity, but it does not change the preset you are using, and the preset can obviously have a huge impact on how the brush would affect a mask.

Let me confirm, it seems the only possible problem is that you are painting with Black foreground color on black mask and thats why there isnt any visible change.

Invert mask (Ctrl/Cmd + I) or invert colors using  X shortcut then start painting.

I might be mistaken, but you appear to be Brushing with a Black brush on a Black mask- Of course Black on Black will not change anything.

If you want the Adjustment layer to show you need to Brush with White on the (black) mask so as to allow the mask to reveal the adjustment where you apply the (white) brush.