First of all don't assume that just because the hens are very young that they wont breed, they can breed under 6 months old and having them in with the other birds may well stimulate them to breed. Second thing, in an aviary breeding situation you need to understand that the risks of hens going into other boxes and fighting other hens and destroying chicks is high, they are best separated and bred as pairs. If aviary breeding you need to be very vigilant and check on them a few times a day if possible to make sure they are sticking to their own boxes. Any trouble makers should be removed immediately. I would have double the boxes that you have hens to avoid fighting as much as possible. If you still want to aviary breed I'd remove the young hens. Then remove the boxes and clean them out. Wait for a couple of weeks and then put the boxes back in to try again. Assuming your birds are in breeding condition they will probably get excited and go investigate straight away. Keep in mind the hot weather though, as it gets very hot in those boxes so once the heat kicks in its best to rest your birds.