Sounds like you’re doing everything right so the only other significant factorsI am hatching for someone else from my town with eggs from his chickens. He drove them maybe 5 min to me. We did have a power outage but only for maybe an hour and I checked my thermometer and it dropped maybe 1/2* in that time. I have a govee thermometer/hygrometer in there that I double checked again before this hatch since the one I just did the air cells seemed a bit small so wanted to be sure it was reading right so that is good. I had one day that for maybe 1/2 hr it shows it was 101.3 but then dropped back down so may be a fluke. the ave temp has been around 99.2-99.6. Since I have an NR360 my govee circles around when the eggs are rolled so the temp fluctuated depending on the spot in the incubator. I can put up a chart from day 7 till today of my temps and humidity.
In June and July I did shipped eggs and from those 2 hatches 28 chicks hatched out of 30 that started to develop. think there were 2 that were DIS at the end but that was it. no quitters the whole time. Then I just did a hatch right before this one (last chick hatched a day before I put these eggs in) for the same guy and 5 developed and 5 hatched.
I checked some eggs a few days ago and looked like maybe quitters and still looked the same today. then I noticed a few others today (I didn't check all the eggs the other day). I can't see veins or movement in them just a dark blob.I left I think 5 in there but don't feel hopeful on those ones. around day 7 or so I have questioned some since the chicks like to hide a bit and sometimes I don't see the veins either, but normally by day 14 I can see them again no problem.
would be age/health of stock and seasonal differences
Maybe some others here can offer some ideas