Casper:
Check the settings for both the wall and the windows & doors. In both cases, if the Floor Plan Display is set to Symbolic, you should be fine, but if this is set to another option, then the Floor Plan Cut Plane is being used to determine the visibility of the elements.
For more information with images, go to Help > ArchiCAD Help, when your browser opens click on the "Search" tab, and input "Symbolic" in the search box, in the results list will lead to detailed information. Also, try "Floor Plan Cut Plane" for a search.
David
