We can of course throw out guesses left and right, but some more info will be helpful.
Temps kept during incubation and hatching
Humidity during incubation and hatching
Were there any dips of the above and if so for how long
The rooms temps and humidity during incubation and hatching
How near to a window was the incubator
How many times did you candle [each time you candle you risk introducing bacteria]
How often did you remove chicks during hatching
That is information needed for hatching. Now to add about your breeders.
How old are the breeders
What is the ratio of male to female
How closely related are the breeders and how far back
Are any molting
Has the weather been constant or has the weather caused drop in egg production
Temp [Incubation]: 99-100F
Temp [Hatching]: 98F
Automatic Egg Turner
Humidity [Incubator]: 40%
Humidity [Hatching]: 60-70% but briefly at 80% after adding water
No dips that I was aware of but a few small spikes during incubation in temp. and humidity - less than a few hours; however it is in a spare room so it is possible there were some usually I have the opposite problem with the incubator running slightly hot previously and have had early hatches so I try to stay on top of it
No window nearby
No candling - the shells are very dark/thick and I haven't gotten a high powered candler yet (I do candle other eggs it is just the buttons/kings that I can't seem to see through well enough)
Chicks were removed once all together- the unhatched egg was removed later for cracking
Breeders
Age: approx. 6 months
1:1
No idea but they probably are related I got them locally
No molting
I wish there would be a drop in egg production LOL I didn't realize they would lay and sit on so many since they have a poor reputation for being broody- the younger female laid 27 eggs in a row! she's sitting on about 10 right now so we'll see how she does as they're due to hatch sometime this week
Didn't considering the fact they might be pretty closely related though. The parents are wild types but they're throwing color mutations, so hidden recessives in there. They are probably from a shallow gene pool. I didn't buy them for breeding, just tossed a few eggs in the incubator to make sure it was still working after being in storage all winter
I'm also wondering if there are temp. pockets in the incubator and I incubated them on the far/right side this time. I've moved both thermometers around the incubator and it seems consistent but I'm not fully convinced. Last batch (full siblings) I incubated them on the left and did not have these issues.
Sounds like your humidity was too low for them to hatch successfully. What did you have it at the last 3 days?? But sometimes that just happens.
60-70% although I did struggle to keep the humidity up and had to add more water a few times. It spiked up higher than I was wanted for a bit. Perhaps that played into it.
I'm so sorry things turned out so bad for you

Did anyone survive in the end?
I have 3 that survived and are doing well! They're very lively and growing in some feathers already.
Thanks guys! There is definitely a curve to learning incubation, and I don't have the most sophisticated incubator. It was a gift and from what I've been reading, still air incubators are finicky. I do have another batch of unrelated [coturnix] eggs and have been watching the incubator closely this time but it has been very stable in both humidity and temp. We'll see how that hatch turns out soon.