I can't tell what is going on without more info. If the layer is white it won't multiply. If it is gray it will. Simple as that.
In AE CS6 3D if you use Ray Trace rendering 3D layers won't use blend modes. There's a warning when you turn on 3D rendering for the first time.
Please include a screenshot. Unless you're doing something really weird setting the blend mode to Multiply will always work.
Sorry, Rick; it had something to do with the way I'd done a making layer; the mask mostly did what I wanted, but somehow disabled the modes for all other layers outside of the mask. I redid the project from the start and it didn't happen the second time. When I have time maybe I can go back and figure it out...