I was hoping someone else would respond to this, as I had the same issue. Over the course of April-May, I incubated a total of 68 eggs. Guess how many I had hatch? 19. Most died within 72 hours of hatching... I currently have 5 that have done very well so far. I cracked open every single egg that didn't hatch, and yes, they were all developed, but hadn't really absorbed the yolk sack yet. Some had absorbed it, but were malpositioned... one was backwards (not sure how this happened as I use an egg turner and it was put in there correctly!) and one had his feet stretched out, pointing towards the air bubble, head curled into his belly.
I use two still air Hovabators and I've never had an issue with them. I even did emu eggs in them (and those hatched just fine!). I kept the top of the eggs around 101-102... I kept track of their weights (I added very little water) and they were on target for the 12-15% range... but they just... stopped before hatching. I didn't see any deformities, they all looked amazing.
The chicks that died shortly after hatching all displayed the same symptoms... they'd be fine, pecking at stuff... then all of a sudden, they'd be unable to walk and would have a seizure or spasms... and then they'd be on their backs and dying. Nothing I did would help them. Some had even learned to eat and drink. Poos looked fine. However, I would look at their bellies... they didn't seem bloated, but they looked bruised. The dead poults had an unbelievable odor... like they'd been dead for a week when they'd just died a few hours ago. The only thing I could come up with was infection from Aspergillus molds or infected navels/Omphalitis.
My incubators were cleaned twice over before I began incubating... so I don't think they were infected from that. I think they were infected from the air around it... I kept my bators in the basement, which flooded a few times this spring (it's unfinished and has a pump and all of that) and the temps in the basement were stable at 64 degrees (which is why I would incubate down there). But it was
very damp in the basement. Inside my bators, my humidity was at about 15%-30% so I don't think that was an issue... emu eggs were incubated in the exact same spot with no problems, but that was January-March when it was a lot drier and the temps in the basement were closer to 55 degrees.
Where are you incubating your eggs?
Hopefully this helps, there is nothing worse that a bad hatch.
PS: Some of my eggs were shipped... and actually weren't able to lose enough moisture... some drowned... so it might have been my humidity was too high or my math was off. I only had this problem with the shipped eggs, though. So maybe my eggs were infected from the hens? I have a hen who's sitting on eggs who's due next week... so we'll see if she has the same issues. My tom is also sitting on eggs, but isn't due for a few weeks still.
