I think I just dealt with this problem in several models. I had no render for WSF but I had photos tagged with WSF. I think this can happen if you upload photos and tag them with a material before going into the settings tab and setting up what materials are for sale. If you set a material as active for sale before uploading a photo and tagging it things appear to be just fine. It's probably a matter of the model database ending up in different final states depending on the order of setting things in all the tabs. (State diagrams, agghhhh!) It looks to me like you tagged the photo with green strong & flexible material and that the green is set as your default material.
Anyway, the way to get the missing render (green in your case) is to maybe do something along these lines.
1. Set all uploaded photos to "select a material" or "multiple materials shown" under the Details tab
2. Go into the sales Settings page and change the default material to something other than green. Save the change.
3. Change default material back to green strong & flexible and save again.
4. Go back to the Details tab and verify that the new green strong & flexible render is generating.
5. Walk around the block to wait for the render.
6. Return to the Details page and select the desired default photo material.
Steps 2 and/or 3 seem to trigger the generation of the missing render.
Or you might be suffering from a different problem, in which case you can ignore this!
This was the original thread where I worked through the problem.
https://www.shapeways.com/forum/index.php?t=msg&th=21450 &start=0&
At first I thought there could not be a render of a material if there was a photo tagged with the material, but then realized a render had not been created.