Some birds take longer than others to get into rabbits, Its hard to give any advice without seeing the bird. You said you have a friend with a Harris, what does he say about the bird.It may require the bird's weight being dropped temporarily or it may need to go up a bit, as i said its difficult without being there.Its just a pity its not late summer with loads of youngsters about. Then your bird could have picked her own prey.It may just be a confidence thing.
ATB
David