When using the import layer from series, Seg3D has had issues loading dicom data due to the interpretation of the z spacing. It seems to be artificially filling the z-direction of the image, maybe due to different formatting/information available in the header. I have tried with two different dicoms and had the same issue with both.
When I have loaded the dicoms into ITKsnap and outputted them as a nrrd, I could properly read the files.