not being funny, but you 100% sure theres rabbits home? if you see them entering and then drop in your ferret, it could be that he's not going deep enough. but usually the scent of a rabbit will get them straight in. I can tell by the way my ferret reacts if theres rabbits home or not.
try checking fields at night with lamp or good torch, it will tell you where exactly the coneys are.
the seasons at and end though.
nothing wrong with a hob mate their steady and reliable. though get him some company, ferrets should be in company.
don't give up, keep at er.