This is cross-posted to other forums, so forgive me if it's a repeat for you. But I am tearing out my hair over this one!
I've created what is meant to be a luma matte, composed of six layers. Three of them are imported from AI and made up of 2D shapes with varying degrees of transparency. Three of them are solid layers with Circle and Turbulent Displace applied. Each of the circle layers is animated to reveal the AI layer beneath it. Then the three "sets" of layers is staggered, and each of the AI layers' opacity is different as well. The resulting effect is various shades of white blobs all "growing" onto the screen.
I would LIKE this shape to reveal a layer below (a pre-comp with its own animation, if that makes any difference). What is happening when I tell the pre-comp to use the white blobs as a luma matte is that it shows through at the very beginning, but as the blobs continue to "grow", it is like they get "too bright" and drown out the layer below, ending up with just the white blobs on screen. In fact, if I put the pre-comp on top, the same thing happens. The white just envelops it, like a light growing too bright and drowning out something in the foreground.
Not sure I've understood what you are saying but since you talk about the white 'drowning it out' it sounds like the Luma matte layeris somehow visible? It should be switched off when used as a matte for another layer
also maybe helpful to know; are you using it as a track matte, or with the 'set matte' effect?