I'm not sure where to even begin troubleshooting this, so I thought I'd put it out there in hopes that someone more knowledgeable than me might have some ideas.  I have a La Femme character with admittedly very complex shaders.  In my go-to photo studio scene, the character is enclosed within single a room with a window behind her, which is itself enclosed within an environment sphere.  My three-point lighting arrangement uses one infinite light (the sun, directly behind the character, doubling as back/rim) and two area lights (key and fill).  This is how the character looks in preview, given this setting.

However, put the exact same character with the exact same materials in most other scenes, and the preview shows her as completely black.  This is what she looks like in the default Poser scene.

Everything always looks fine when rendered.  Only the preview seems affected by whatever is going on.  This isn't a major problem for me personally.  I just activate mesh view for posing purposes and/or rely on low-quality draft renders if I need to preview the character fully.  But this is a package I'm hoping to sell, and the testers have stated that this is a deal-breaker.  So I would greatly appreciate any advice anyone can offer.

My best guess right now is that there are just certain combinations of shader complexity and lighting arrangement to which the preview renderer says, "No problem!" and others that essentially make it go, "Nope!  Not happening!"  I'm actually hoping I'm wrong, though, because that would almost certainly mean there's little if anything I can do and still get the effects I want (short of elaborate work-arounds like conforming a duplicate figure with much simpler materials, making the main figure invisible, and swapping the visibility settings before actually rendering).