Hi,
It sounds like not only is the pair too young, but they need to be rested. You can’t leave the nest in the cage year round. After each clutch, it should be taken out so the parents will rest for 4-6 months. Why did you allow the male to bully a chick until he killed it? Did you leave the chicks with the parents for too long? The nest should be removed as soon as the chicks leave it, and the chicks should be moved to another cage as soon as they are eating on their own. Otherwise the parents will pick on them because instinctively they want the chicks to leave and find their own mates and territory. If a hen keeps laying eggs over and over, she will end up dying from laying too many eggs. I would remove the nest and rest the pair from breeding for at least 6 months. Longer if she has been laying a lot of eggs. You should limit their light to 8-10 hours a day to discourage egg laying right now. Cover the cage early each evening. Move the cage around the room weekly to discourage nesting. If a hen has been over bred, laid too many eggs, is too young or too old or not on a good diet, these are reasons the eggs may not be fertile. If you are only feeding seeds, then she is not getting much nutrition. You should be feeding them a finch pellet or granule, leafy greens and chopped veggies. She also needs a soft cuttlebone. When it is time to let them breed again, offer a daily egg food – either a dry commercial egg food or cook an egg with the shell washed, crushed and cooked with the eggs.
Thank you for asking Lafeber,
Brenda