This is quite common with collie lurchers, they try and herd the rabbit instead of picking it up!
keep retrieving with a dead bunny, and if you can beg/ steal/borrow a lure machine tie the rabbit to that and gee the dog up to catch it, if not use a piece of string on a dead bunny and run round letting the dog chase and catch, you might want to do this out of sight as you tend to look insane!!!