If it's happening with birds you are breeding yourself then there may be a genetic component. In that case I would try to follow which eggs are from whom and not hatch from that particular bird again. Sometimes parents are healthy enough for themselves, but not enough for good chick developement. Might want to look at what you are feeding also, check mill dates on bags and make sure your feed is always fresh (vitamins deteriorate with time), and make sure that too many treats and other stuff isn't contributing to possible deficiencies or low levels of nutrients in some of them. Just like people, every one is a bit different.