Layer Mask not working properly-help!

I have an action that I created in Photoshop CS2. I used layer masks to be able to edit each layer. All of the masks work fine except for one. On this one layer (we'll call it layer 1) the mask works fine at first, but if I click onto another layer and then come back to layer 1, the mask won't work!

Instead of painting to reveal the layer beneath it, the brush starts painting black.
Why would the layer mask work fine until I click on another layer?

Thank you so much!

Sounds like you have the layer targeted, not the mask.
Look at the payer palette and make sure you "target" the mask
(by clicking on it) before using the brush.