External faces issue when adding objects to an extrusion


This seems so simple, but I can’t make it work. I have extruded a rectangular frame. When I try to add parts to the side of the frame, the inspector tells me there are external faces. When I tell the inspector to fix the problem, some parts are removed from the drawing. . I am using SketchUp 16 but get the same results in SketchUp 17.
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