Have you tried him in different locations, maybe the place you use him will often end in a dig no matter what ferret you use, I,am not telling you how too do your job, but were you quiet when setting up as they often need digging out if there disturbed before hand. Some of the places I go i know before hand will end up in a dig, its down to the warren. Most ferrets will scratch the back end off a rabbit when its in a stop end and eventually kill it if left long enough its just down to where the rabbit bolts to, wether it goes into a net or a stop end. The ferrets not there for a chase.