Bee, that sounds like dead during incubation in the first few days of what would be normal development. Fat Cochin is probably doing his job just fine, but the temps of the eggs were probably off.
I seem to recall reading that in most DIY incubators they go for a slightly high internal egg temp at the top, and a lower one at the bottom. So an internal egg temp at the top of like 101 and a little lower at the bottom. That is why still air incubators run at a higher temp (101ish) than fan incubators that keep the whole incubater one exact temperature. So I think you are on the right path for Test Nest #2!
I seem to recall reading that in most DIY incubators they go for a slightly high internal egg temp at the top, and a lower one at the bottom. So an internal egg temp at the top of like 101 and a little lower at the bottom. That is why still air incubators run at a higher temp (101ish) than fan incubators that keep the whole incubater one exact temperature. So I think you are on the right path for Test Nest #2!