A .swf won't behave the way it should when opened within another swf.. ?!

Are there known common problems with loading .swfs modularly, ahem, within other .swfs? are there common precautions that should be taken?

For example I’ve got one that has a menu that appears and dissapears upon rollover, with a neat multi-MC mask effect. The effect works when I test the .swf on its own but when loaded in an external .swf, everything works fine except for this mask effect. uugh