Genetics is weird..
I want to ask you something. How many chickens do you have? And your broodies are great moms I see, ours are terrible this year. First hatched 8 out of 15 eggs, but she broke two eggs and the rest she killed while they were hatching. One egg was empty. Another one had also 15 eggs and she broke 6 eggs. Eggs weren't grandma's and only three chicks hatched. And the broody threw one out of nest and was picking the other two. So we are kepping all 11of them in a brooder. The one that is sitting one week now, also had 15 eggs and she broke 2 already. We are freaking out because of them.
Only 52 chickens.. no idea how many chicks.. probably in the 150ish range.
Yeah I said only 52.... for many years I always had more than 300 chickens, not counting the babies. And 100+ peafowl on top of that(only have two peafowl right now). So 52 seems very few to me... ha!
It is the peafowl that helped me figure out and pick good setters and how to use broody hens to their best. Peafowl eggs are very difficult to hatch well in incubators... they hatch out far, far better under peahens or hens. Part of the reason I had so many chickens before.. needed to maintain a number of setting hens to hatch the peafowl eggs.. which require 26 days to hatch.. so, try to imagine how many broody hens are necessary for the entire peafowl breeding season, ugh.... On the other hand, I was quite successful at producing hundreds of peachicks every year and they did sell for good money.
A couple things to help with successful hen incubation:
Plenty of room in nest box, but not too big. If the nest is very big, it is possible the hen may try to shift positions within and sometimes eggs get lost/chilled in the process.
It helps a lot to have nests that are identical and preferably easily move-able. I use covered cat litter boxes, most of them are solid blue. I just pick up the whole box with the hen still inside.. no problem.
Isolate a broody hen, completely. Letting other hens have access to her nest is asking for trouble- a lot of eggs get broken this way.
I had rows of cages and small pens expressly for the purpose of isolating broody hens. Only one per section. If this is not possible, I would have gone with something similar to a stack or row of brooders against a wall, except adult chicken-sized, with enough space for the nest and for the hen to exit the nest for a bathroom break. Like rabbit cages if that makes sense? The hens above were both moved to an empty pen. They would have hatched less chicks if I left them with other hens that were still laying.
Make sure they are healthy, free of mites and lice. Dust or medicate if they are present.
Use nest materials that hold shape well. Especially material the hen is able to arrange around her to make a close fitting cup. I use fresh straw or tall stemmy weeds from around the yard. IMO shavings is terrible for this purpose. I think this really makes a difference in how many eggs a hen can successfully incubate.
IF an isolated hen still breaks eggs.. she may simply be 'heavy footed'.. give less eggs in case the hen is not really heavy footed but likes to clamp down and squeeze the eggs together too hard.. these hens do better with fewer eggs than the maximum possible. Sometimes you can tell these hens by if they forcefully press down and make it difficult for you to reach under.. the best ones don't do this- very easy to reach under and pull out an egg without any fuss. The two black hens in the pictures are like this so I gave them honestly, too many eggs but they pulled it off. With a 'clamp down hen' I would have given the same size only maybe 10 eggs maximum.
The other possibilty to consider for broken eggs- were the eggs too thin shelled? Too different sizes mixed in same nest? A small egg in a batch of big eggs will lead to broken eggs.. it is like they 'poke' in the other eggs when the hen rolls then rocks on them before settling down.. best if they are the same size and shape. You can still hatch thin shelled eggs as long as they are not abnormally thin by letting the hen sit on far fewer of them.