Thats really interesting..are the eggs shipped eggs? local eggs that you picked up?
There might be something in the environment , boy thats tough..if they were shipped eggs, try to do a hatch with some local eggs that you can just pick up from a breeder. I have an APA judge freind who literally drives across the US with a special van and picks up birds and eggs that he wants..I thought he was absolutly crazy until we went through all of this with that rare breed.. I get it now..he said it will always be too much of a gamble with the post office.
A freind and I got many shipmens of a rare breed from several different breeders and we would always get 2 or three to hatch and the rest no matter what we did had trouble..might have been the inbreeding due to the rarity of that breed, had better luck with breeds other than that rare one. and we keep the embryo absolutly still and upright for up to 7 days in incubator to try and give it strength later on..worked for certian breeds but that rare one never did well shipped..
Another local freind gave them a batch of light sussex and they all hatched but 2 out of 20.. same incubator that you have