BoneJ Moment of Inertia issues with ROI

I’m new to BoneJ and want to use it for research and teaching. I am using this program to calculate moments of inertia and cross-sectional properties.

Problem - when I have a DICOM stack and I select ROIs for the bone of interest, when I run the Moments of Inertia routine it often either samples some other area or I end up with nothing at all. Any ideas why this might be?