Have you tried putting some hay in a pile at one end of the litter trays? That usually works very well.....or a favorite food, like parsley. It may take them a few days to find it...for all that they are supposed to have such a great sense of smell, I find that my buns often can't find the carrot i put right in front of them. They are limited in their frontal vision..they have a blind spot directly in front of their nose, which is why the often jump and run away, growl, or charge at you if you come at them too fast. Keep trying, patience is the answer, I promise!