Get her speyed, female rabbits are very prone to cystic ovaries and ovarian or uterine cancer meaning that it is hard for these ones to fall pregnant. However because you keep mating her you are putting her through the stress of phantom pregnancies, where they believe they are pregnant because they were mated but actually are not so go through the ritual of nest building etc only to find that no babies are coming. If she were speyed she would not have these phantom pregnancies and start building nests.