Some renderers don't render back faces even if they are painted with materials. This is one of the reasons it is a good idea to stay on top of the face orientation in your model as you work.

fwiw, there's a thread similar to this from last year

http://sketchucation.com/forums/viewtopic.php?f=11&t=43950

in it, i attempt to offer an explanation as to why this stuff happens.. (point being, it's not a random occurrence and has actual reasoning behind it)