Surprisingly i can't find any mention of this anywhere on the intertubes - very often I have a placed (photoshop/tiff) graphic with transparent background overlaying (on top of) a spot color area, the spot color prints out completely different from how it should be (and how it is on non-overlapped areas).
I've tried everything i can think of, including making an opacity mask to 'hide' the supposedly transpararent areas of the placed grahic. I know a clipping path might work, but some art is too complex for that.