Trouble with obj files

I'm using the built in obj loader supplied with java 3D. I have an example object file of a cube which I have confirmed works in many programs however in my application some of the faces do not get drawn.

eg. When I look from top down I see a cube however when I look from bottom up on the model only 2 faces are drawn. Is this likely to be an issue with my lighting?

[381 byte] By [Mattyw83a] at [2007-10-2 12:55:13]