i think that masks in flash will reveal what you are essentially "covering". its like you're making a window with the artwork. you cna make any layer into a mask by right clicking it and choosing "mask". you can only see the mask doing its thing if both the mask layer AND any layers under it are LOCKED. unlocking any of the layers will turn the mask "off" while you are working. if you step out of a symbol, or render out a SWF, it doesnt care if its locked or not. this only happens whe youre working.
you can animate a mask to reveal parts are you wish. the simplest way is to keep your mask as artwork, and just use a shape tween to give it simple animation.
hope that helps.
To any Non-flash artist, that would make zero sense. I understand it completely, and I still hate flash for doing that.