Ron,
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.
As a note to the newbie hatchers & others -- last year after several; failed hatches my DH bought me 12 yes 12 of those little outdoor thermometers so I could test temps in diff. places "looking for hot spots" etc.
several weeks and lots of dead eggs later, I finally put them side by side & discovered a 10 yes TEN degree difference in readings across the 12 of them
So, yes calibrate, or at the very least realize even the $20 fancy one w/ remote readout could still be way off. I also found candy/meat thermometers tended to be more true in readings as far as low cost options & you can slide the tip into the air hole (unpluged plug hole)s of LG & I figure other type bators as a way to further verify or cross check readings (my "fancy" $20 unit that gave temp & humidity remotely apparently went wonky during my first lock down , ironically I think it could not withstand the heat&/or humidity & itself overheated, so the more manual candy/meat thermometer was a trusty backup when I needed a reality check on my literally *blinking* "fancy" one). So I hope that info helps someone, and saves some time/money/effort/eggs.