[Closed] Problems going from GI to FG

I’m trying to light an interior scene using photometric web lights but am having all sorts of problems.

I think I’ve found a reasonable GI solution using just a ‘matte finish’ A+D material applied to everything, but as soon as I switch on FG the upper half goes PITCH BLACK!!! …See below.

Why does it do this? …I just cannot find a way of illuminating the upper half of an interior, and am close to tearing my hair out!

Any help appreciated.

PS. I’m using Max 2010 + Mental Ray