I modelled the first floor, with the windows. Then I copy-paste to generate the other floors; now every floor have the windows. Then I close and re-open sketch-up, and I open the openstudio model in skethup. I can see the external walls of the whole building, I can see the windows of the first floor, but can't see the windows of the other floors. Why do they disappear if I close and re-open sketchup?