Ugh, I’m sorry. It sucks to lose them at the end like that. In my limited experience, what I’d look at is:
How were the air cells? Large/small/just right, were there any deformities in the air cells? (detached, saddle, too far to one side)
Were the chicks big or small for the size of egg and air cell?
Were you using the temp/humidity readings on the incubator, or did you lay a calibrated thermometer and hygrometer in the incubator? I ask because often the readings are inaccurate on the machines. A manual “people” thermometer is great for spotting any temp spikes that you might not otherwise catch.
Sometimes it just happens. There might have been something wrong that you couldn’t have done anything about, that you couldn’t see, etc..
I’m sure someone with more experience will chime in. I’m really sorry for your loss. I also found a dis/quitter in my hatch this morning. It’s frustrating and disappointing.