Pretty sure you're right! Thanks for the input. One of the things I love most about keeping chickens is that I learn new things from them and about them every single day :)
The eggs were from a few different groups - one roo is just under 2 and the other is a year old. The hens and pullets are similar ages.
In particular, the eggs from the year old roo and his flock have had almost 100% fertility/hatch success this spring, so it was startling that they were quitters.
I'm still learning about broody hens and this one's been hard to describe in a way that brings up any search results.
I have a hen - a pullet, really, about 10 months old, but I thought I'd indulge her in brooding. I put four eggs from mature hens known to have high fertility and healthy hatches...