It might have to do with how you are editing the model. I believe when you import, you want to double-click and do new inherited scene. Then save the scene with some other name. After this you may have to delete the original version from the tree (you can leave it in the assets, just not on the screen) and then drag your new inherited scene into the level.
Depending on how you import the model, you may, instead, need to choose open anyway, then save the scene as something else and do the rest of the steps above. That may or may not be your issue, but it has confused me before so that might be it.
