Though it is hollow, it still has some thickness to it, right?
Consider debugging a simpler version of your scene.
Check to see that lightmap is attached and being used.
One thing we do in class, just to adjust the top sss layer and know we're in the right ballpark, using the light map, etc. ...
Turn all weights to 0 except the front sss, ie leave
Epidermal Weight, (or even set it to 1.0 temporarily)
but zero out
Diffuse Weight (that's direct diffuse (lambert model))
Overall Weight for Specularity
Check your lightmap connections if you don't see anything. Are you using the indirect light for your light map?
Then, adjust your blur/softness with the Epidermal Scatter Radius. It's easier to adjust if you have a sharp shadow from the direct light.
After that, we adjust the Subdermal and Backscatter layers to have increasing softness, larger radii relative to Epidermal.
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