Something similar happened about a month ago to my first-time broody hens also. There were two brooding the eggs, and one was VERY attentive. Only one chick made it out, two pipped slightly, one pipped almost all the way out, the rest were bad. My theory is that partly because they were laid in the very cold weather, and partly because there were maybe too many to be completely covered all the time, the weather was a factor. We all know that chickens rotate the eggs, and when they can't quite cover them all, the ones that spend more time on the edges can be damaged in various ways. I think they may have been damaged just enough they didn't have the strength to hatch fully. Also, a couple had been thrown from the nest. I feel sure some first-time broodies (especially if they're outdoors even 'though protected well) have seen mice and mistake the sounds of hatching and newly-hatched look of their chicks for mice. Had this happen with a guinea one year. She killed the first three or four until she seemed to realize what they were. After that, she was VERY protective of them.
I have trouble NOT interfering, and would probably have started in when I could no longer hear them peeping. I've "saved" more than I've lost, but it is really tricky if you start too soon.