Ugh. I checked the nest while the broodys were on their break late this afternoon, and there was another dead chick. This one was dry, and I couldn't see anything obviously wrong with it. This would be day 20, which isn't impossibly early. There were also four newly laid eggs in the nest, so I wonder if the poor thing got smothered when the other hens pushed in to lay their eggs. In case that was the problem, and I should probably have done this before, I took some leftover hardware cloth and propped it up so that the section of the coop they're in is fenced off.
I'm trying to be philosophical about it, if I hadn't have let the broodys have these eggs, they would have been eaten, and if the chicks that didn't make it were roos, they'd be going to freezer camp pretty soon. But even so, its disappointing.