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If I understood you correctly, in troubleshooting low hatch rates, if the unhatched have developed say to day 17 or 18 & then died so when you open & look in you find that they have not absorbed the yolk at all, & often not actually turned right side up as they would need to in order to pip, but they appear in all visual ways to be correctly formed so it looks like they should have continued those last couple days, & you find that you have a large number or even all your eggs do that then the issue is ? (a) slightly low temp or (b) lack of oxygen ie not enough airflow into the bator in the final week.
That would explain so much about the failure of my DIY bator last year!!
I've long since upgraded to the a LG incubator but even so this clears up some puzzles for me.
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