Ah.... when you say "inside", how exactly do you mean?
For example, I have a building and all its walls, floors, roof, built as a single entity. Then, I have the windows built as separate entities, and then in my main level, I have the building entity loaded, and then I have the window entities loaded and placed in the open window frames of the building entity. Is that what you mean? Or do you mean if I "built" an entity on its own that had another pre-built entity included in that build, and THEN tried using that combination in my level? If you mean the former example, then I'm not sure 3DGS is going to work for my project if I can't load pre-built pieces of my building like that, but if you mean the latter, I might be able to track down the extra entity in question and revise my build.
Thing is, it all works fine in preview mode. This really is a stumper.