Problem with ceiling lightning

Hi! I’m doing an architecture project, and as a first step, I imported only the basic volumetry for doing lightning tests (walls, floors and ceilings). But I have a problem I don’t know how to resolve. The ceilings of the house are darker tan the walls, and I don’t know why. They have the same material, same UVmap (a basic 1x1x1 meter from 3Dsmax) and same lightmap density. I checked the face normals and things like that, but I can’t solve the problem. The light is the predeterminate (skylight and light source), I didn’t change anything about it. What can I try? Thank you.

PD: I did two photos so you can see the problem. I want the ceiling with the same illumination than the walls.