Mesh Shadow Problem

Hey folks, I'm trying to use Grasshopper to generate fabrication patterns for limestone, based on a 3d laser scan of a scaled down model.  The mesh is obviously not watertight, and has quite a few points/faces (though substantially reduced from original) - are either of these what is keeping Grasshopper from being able to project the mesh shadow on to the bouding box planes?  The definition I wrote works fine on other meshes, including some rather large ones, but for some reason it simply won't do anything on this one...  Ideas?