Hi Steve, can you please post the .3dm file? It is hard for me to guess what might be wrong just by looking at images only.

But it appears that likely the problem is that your final shape there comes to a sharp point at the tip instead of being smoothly rounded at the tip.

That would make the offset of it not touch at a point anymore unless it was extended, which I don't think that the sheller is able to do in this case.

Typically for thickening a surface into a solid, the sheller tries to create lofted surfaces between the edges of the original and the offset, but in that case there is no edge on the collapsed pole point to match with the offset surface's edge, so that probably confuses it.

You can probably produce the shell manually in this case by using Offset instead.

- Michael