@Bradley
I have found with Lofts where an End Face is beyond a particular angle Shell does not function.
Can it be assumed that you have made several attempts to Shell the Lofted Body?
Is the Circular Plane at an angle to the other Planes?
Have you tried Shell from the Circular Plane?
In the following shapr.File the Leftmost Image Top Face is at 10º and will not accept Shell from either end.
EDIT: The above statement has proven not to be true every time. Revisiting the same file the Shell was found to be successful. The oddities with the Bodies created have been made known to the S3D Team, this aspect may be cleared up at the same time?
The Middle Image is at 5º and accespts Shell from either end. Use the stacked Planes to experiment.
LoftNilShell.shapr (340 KB)
If this does not help may I sugest to download your shapr. File to Dropbox and publish the Link to enable us to access it here?