with these kind of things, it always helps if you post an image.

but let me try anyway...

shadow catchers are used in conjunction with composited live footage or backdrops. basically you model a primitive object of something that appears in your backdrop photo, line it up in the right place, and set it to be a shadowcatcher. it will render invisible, except for the shadowed area, which will appear to fall on the objects in the backdrop.

hopefully that makes sense.

since you've modeled the actual room, you don't want to be using shadowcatchers. objects in carrara will always accept shadows, as long as the objects or lights that would create said shadows are set to "cast shadows."

hopefully this helps produce more desireable results.

-pix