It's hard to say. It just happens some times. If your chicks that made it took too long to hatch or have crooked toes or are too fluffy, these can be indications of a problem with humidity. If the temp spiked right before hatch, that can kill without any pipping. Temperature is more important than humidity, or so I've heard. It's hard for a hen to overheat the eggs, but she might have left them out in the cold too long. Also the quality of the hen making the eggs can affect the fitness of the chicks. Moulting can cause a hen to lay poor quality eggs that may develop embryos, but not be able to hatch. There are so many things that can go wrong. Most people say the best you can expect from a clutch made of 100% fertilized eggs is an 85% hatch rate. I've been told myself, that out here in the desert were I live, it's more like 50%, both artificially and naturally hatched. My experience so far has been well below that myself, so don't feel bad. But, generally speaking, mom does a better job than the incubator where ever you live and what ever your temps are. For example, for my recent natural hatch local temps here were 102+. Oh yeah, and chicks often die right before hatch time.
