I had 7 eggs due to hatch today, 5 hatched last night, 1 more had pipped this morning before I left for work, there was a big hole when I returned from work but not zipped, I removed and placed back in the bator on a piece of warm damp kitchen towel as I was worried about the membrane drying out, chick was fine for 4 further hours and then nothing, no more progression and no peeping. Just checked and it had died, yoke sack was fully absorbed, beak seemed further away from pip than I had expected, chick was moist but not sticky. Hate losing any chicks but this one was so close! I was hatching in a Brinsea Polyhatch still air.
No telling why, some just do. I have had them partially zip and die and some that piped, zipped and hatched and then died.. None of us here hasn't experienced it and nobody can give a reason ... It just does... Sorry
Sorry for you loss… It does suck a bit when a chick comes so far, only to die. Sometimes the chicks are just weak, unable to hatch by itself, sometimes the cause is an incubation error or the problem goes back to the egg and breeder flock. This article breaks down some of the most common known causes for chick and embryo mortality: https://www.backyardchickens.com/a/egg-failure-to-hatch-diagnosing-incubation-problems Hope it helps!