self illumination map problem

[cakewalkr7]

I am working on the lighting for a scene and I'm using mental ray to render. I have final gather turned on. I have created a cone as my lighting housing, turned it to editable poly and set all faces to a material id of 2 except for the main interior face which is set to 1. I then created a multi material with channel 2 using the arch&design chrome. In channel 1, I have a standard material with self illumination color checked and set to full white. When I try this same material in a new scene, it works fine. But in this particular scene where I'm actually using it, it doesn't seem to illuminate very well. If I uncheck it it's black, and if it's checked, it's a cream color... but definitely not a bright white like I would expect. What would cause this? Thanks.