has it just started doin it, if so have you looked at feet or legs for injury`s etc,
was the ground soft or hard with frost etc, try it on a good soft field on a cupple ov easy rabbits and try to end the night on a kill keeps the dog keen for next time it goes out,
never give up on a dog till you have tryed every thing you can, if still no good use it for ferretin or bushin, just a few things i would try befor rehomein
hope all turns out ok mate.......lamp