This may sound like a ridiculous question, but is the shutter open?

Also, is the ground glass really ground, not just a clear pane sitting there as a placeholder for some strange reason? It doesn't seem like you should be able to see the back of the lens through a ground glass, unless you're putting a lot of light through it from the outside.

If you have a lens in an open shutter, a dark space, and a ground glass, you should be seeing an image---I can't think of anything that would interfere.