Brush tool won't paint black in masks of adjustment layers

this worked until yesterday. I didn't install any software on the pc or change any settings that I can think of. I'm using photoshop cs2. I've rebooted the computer several times and I've quit/restarted photoshop several times.

when I create an adjustment layer, I cannot paint black in the mask area. I can fill with black and paint white. if, after filling black and painting white, I press D (to swap foreground/background colors), I can paint, a paintbrush thing shows up in the history, but it won't change. if I press D again, and paint some more white, it shows up in white. likewise, if I choose black another way (eyedropper, setting rgb to 0,0,0), it behaves the same. likewise, if I don't start out by filling black, and I just try to paint black on the white mask, it behaves the same (doesn't work).

if I create a new layer, I can paint black on it.
If I switch to the pencil tool rather than the brush tool, I can pencil black into my adjustment layer masks.

any suggestions?
or what info can I get for you so you have a good place to start?

thanks in advance - this is killing me.

Check all your brush settings and see if you have turned down the opacity, or any other weird changes.