How old is she mate? I wouldnt go working her to much on the old mice when young, seen many a good dog throw the towel in when working them to young & make sure you have some peicilln in to, they can give a very nasty bite, and never double them up sure way to feck them up in the long run.Best thing to do is start them off about 7/8 months on ants then move them up to mice about 14 months old on. Steady is the key to a good terrier. pm me if you need anymore help cheers christian