Hello Paul, A possible reason why your polygons disappear in some angles is basically cause you haven't unified the normals of your model and make them double-sided in the balconies and the stairs (you do that in your 3D modeling software). It that doesn't work you should make another polygon facing the oposite direction where the maps disappear.
I hope this helps. Cheers, Ivan Moreno http://www.thefloatingwall.com/
