@tig said:

I think your problem is as you suspect...

The two surfaces are incredibly detailed, so the chances are that you'll get tiny differences in the subtraction results, where tiny edges get auto-omitted and no face forms - rendering the result non-solid with 0 volume...
Since cut-and-fill is in reality an inaccurate science on site, can I suggest you try with a less subdivided mesh to start with...
The rogue out-liner might also suggest that the two forms' skirts are not coincident, which can have awkward results...

You will get the equivalent result [non-solid 0] if you use the basic 'solid' subtraction on the two forms...

Great feedback thank you! I got it working now! The most important things were:

taking down the detail of the mesh. I got it working with approx. 4' grid which is a good accuracy range for earthwork I think.
closely analyzing and repairing topo line connections with TopoShaper
I also found that it's okay to grade the edges of the mesh with the artisan brush as long as you hold shift to lock the vertex adjustments vertically, so that you don't mess up the mesh boundary.

Awesome plugin, thank you so much!

@tig said:

As explained in some earlier posts, it is possible to contrive two solid surface forms which when subtracted do not result in solid object [a limitation of SketchUp itself], so then the result has no volume - simply because it's not a solid.
You can get this result using the native tools rather that this specialist version of it - which actually uses the native tools via the API...

Obviously start with two solid existing and proposed groups...

To try to avoid these kinds of issues then ensure that their skirts coincide exactly.
The tool has a built-in skirt adder tool...

Next ensure that the existing and proposed groups have difference in their surfaces that are sensible.
A few mm difference in the elevations of their surfaces' faces is likely to result in a subtracted form with holes in it - because SketchUp can't create edges < 1/1000" long during the intersection and 'solid-tools' operation, so then faces that would have used those skipped tiny edges fail, and then the resultant form is not a solid and therefore it has no volume.
Remember that you are considering earth movement here, and accuracy to a few mm is not practical anyway, so be 'bold'...

Hey, @Tig this sounds exactly like what I'm experiencing with my grading project. Solid inspector and entity info show solid groups for EXTG and PROP, but not getting a solid result from the cut n fill calculation.

Hoping you would be able to take a quick look at the file attached and give any guidance on how to avoid these tiny discrepancies.

Thanks!

CUT N FILL TEST.skp