I know this doesn't sound like much help but sometimes they simply die without you ever knowing why. I had that happen with my last broody. One little chick pipped, zipped, and died. I found it half in half out of it's egg. I suspect momma waffle stomped it moving about with the already hatched chicks. I felt bad. I had the incubator going. I should have recognized the warning signs. This time I won't let it progress. IF momma is getting restless with already hatched chicks, the remaining eggs are coming out and into the incubator.