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).

Are you using Cycles shaders? If so, make sure to set them for Superfly but also have a PoserSurface shader which is valid and selected for Firefly. E.g. have a diffuse map plugged in with a value of 1.0, not 0.

Beyond that, might be helpful to post a picture of the shaders you are using.

I am indeed using alot of Cycles nodes, at least.  Here's my attempt to show as much of my shader as I can at one time.

Skin_Hair_Blend_Diff plugs into Alternate_Diffuse, and Skin_Hair_Blend_Spec plugs into Alternate_Specular.  The Skin_Diffuse_Map is plugged into main Diffuse with its value set to 0.  The stuff on he right is just doing math on input maps to generate a final hair mask based on user settings.  I also make heavy use of customized compound nodes to aid in organization and the reduction of clutter.  The main selling point of this product is that it enables the user to control various aspects of the shader (skin color, eye color, hair color, presence or absence of freckles, etc) via parameter dials instead of fixed presets.  That's why it has to be so complex.  The default dial settings produce this when rendered.

As it stands, the buck stops with a PoserSurface node activated as both a SuperFly Root and a FireFly Root.  I'll experiment with using separate roots and see if that helps.  Thanks!

luvenis, do you have any lights in your default scene?

Yes, there are lights in every image.  For all but the very first image, I just loaded the character into Poser's default scene, which includes the default lighting arrangement.

It's likely the shader is too complex for the preview to display. You might need to turn off hardware shading in the render settings preview tab. Another option might be to add another poser surface root node with just the image maps plugged in. That can be iffy as poser might get confused about which node to preview.

Cycles doesn't preview. Cycles materials show up in all white.

Cycles materials that also have a Firefly root node (e.g. a PoserSurface root) will preview using the Firefly root, not the Superfly root, even if Superfly is the selected renderer. But that's only true if "enable hardware shading" is unchecked in the preview settings. If it's checked, they preview in all white. That's with OpenGL. If you pick the Sreed preview option, it always uses the Firefly root regardless of that setting. (It's possible this has changed in newer updates? But I haven't noticed anything.)

If you are in one of the scenarios where the PoserSurface root is being used, it can give an all-black image for some invalid situations, e.g. you have a Diffuse_Value of 0. You might be thinking it has something to do with your Cycles shader nodes, but it doesn't.

I would test the skin shaders under different lighting conditions, the default Poser lighting is really crap.  You will find that one set of lights will work very well and then a different set completely fails to deliver the best results.  I like the concept of controlling various shader settings through parameter dials, that's a novel idea.  Be careful with using Cycles nodes with the PoserSurface, I have seen variability between similar nodes when used that way.

Success!  The key seems to have been in ChromeStar's recommendation to have two separate root nodes and assigning each to a different render engine.  A part of the solution was also taking up Hborre's advice on not plugging Cycles nodes into PoserSurface roots.

So here's what I did.  For each material zone, I plugged my original shader into a CyclesSurface root node and disconnected it from the PoserSurface root node.  I then plugged very basic diffuse and specularity into the PoserSurface root node.  Next, I designated the CyclesSurface node as the root for SuperFly and the PoserSurface node as the root for FireFly.  Finally, I made FireFly the current render engine in my scene, and voila!  While I was at it, I took the opportunity to add a preview shader for the eyebrows, which didn't even exist in my original workflow.

It's not an entirely seamless fix, since it essentially requires switching between render engines whenever I either start or finish rendering an image (e.g. if FireFly is active for preview purposes, I need to switch to SuperFly for actual renders), but it's much less hassle than anything else I thought to try.  For the sake of potential FireFly renderers, I wrote a Python script that will go through the figure's materials and swap the root node assignments.  So FireFliers can use SuperFly in preview and switch to FireFly for rendering.

The best part is that, even if I don't switch to FireFly in preview, the figure displays as light gray.  Although it's still textureless, it at least makes shading visible, which is still an improvement over a dimensionless black blob!  Thanks for the tips, everyone!

And Hborre, I'm glad to hear that the idea of dial-controlled shader settings intrigues you.  Now I know I'm not completely alone.  Maybe it's just me, but it has long seemed like a rather obvious step to take with the advent of animated shader nodes and master/slave parameter relationships.  Frankly, I don't know why fixed presets are still the standard that they are!