Cast shadows of 3D objects won't render in PS CS6

I have a photo of some objects that I'm trying to composite-in shadows cast by 'invisible' 3D models of the original objects and I cant get the shadows to render.

The scene is set up as follows:

• The first layer is a photo with a vector mask revealing transparency in the background.
• The second layer holds seven 3D objects imported from .obj files, then merged down to a single layer (and scene) with a single infinite light (at least for now).
• In the properties for each objMesh object, I have UN-checked 'Catch Shadows' and CHECKED 'Invisible' and 'Cast Shadows'.
• The background layer is white.

A .jpg saved out of the scene with a preview of the shadows looks like this...

...but when I try to render even just a small marquee selection of the scene the shadows won't render... it just comes out white.

I've tried casting shadows on the 'ground plane' of the scene itself (as in the image above), and using an actual plane (shadow catcher) snapped to the ground plane of the scene with both 'Catch Shadows' and 'Invisible' CHECKED and 'Cast Shadows' UN-checked in it's properties, but that didn't work either.

I just did a similar (yet much more simple) image as a trial run with all the same settings and the shadows rendered fine... didn't quite get the ground plane matched exactly, but the shadows rendered...

I've been beating my head against this wall for a couple of days now and I was hoping someone here might have some good advice?