I know this post is super old...but I have to add that the chicks most likely died from other hens having access to the eggs. This happened to me with the first egg that hatched under my first broody hen. I quickly closed off that part of the coop to the other chickens and 12 more eggs hatched and survived.
I recently had another bad experience with other hens being near the nest. I had one broody hen in the coop for two weeks and during that time I let the other chickens have access to the coop. I marked the original eggs with marker and took out the new eggs the other chickens laid from the nest each day. I planned to close off the coop to the other chickens once 21 days was near. Before that happened another hen got broody and laid on a nest about a foot away (the coop is very very small). I shut them both in and they peacefully laid on their respective nests for a week. The first hens chicks started hatching and all was good for two days while the hen was still sitting, waiting for the rest of the eggs to hatch. Then some of the chicks started going under the other hen...which she accepted just fine and I let it go. After a while I noticed that the first hen was attacking some of the chicks. I wasn't sure what was going on. Long story short...she thought that the chicks that were the same red color as the other hen were not her chicks (it was a shared nest so she had all different kinds of chicks) and she wound up killing two..it was really sad. She was attacking a third before I intervened and separated them. The third chick was unharmed and I put it and the rest of the unhatched eggs under the second hen. Two more chicks hatched overnight and were with the other still sitting hen for a day and a half before I reluctantly took them away to raise inside so the mama could concentrate on the week old eggs. The first hen, a Columbian Wyandotte, is a terrible mother. Her six remaining chicks are a few weeks old now and are doing well...but stepped on them a lot in the beginning and always scratches all of their food out of their bowl, mixing it with the dirt...So annoying. So far the second hen...a red sex link I think...is a great mother...she has seven new chicks. A third hen just got broody...so the saga continues!