Maybe you can do a test run. Take another set of eggs from your hens and roosters and put them in an incubator. See if they hatch. If they do..., then either your hens need more practice being broody or maybe there is a bacteria or other microbial/ environmental factor getting the eggs as they are being brooded. If they don't hatch..., then dig deeper into the genetics, possibly bacteria in / on the eggs, ???? Just guessing and trying to think outside the box here.
For some unexplained reason, my post got stuck into Melissa's post. Sheesh. need more coffee!