Quail eggs - late hatching

[Farm Lady Fanci]

I have 43 quail eggs incubating. I did dry incubation with temp at 99.5 and humidity stayed around 30-35%. I am using a nurture right 360. There where no incubator issuers or drop in temp. The temperature was double checked with a back up thermometer. They went into to lock down and the humidity was raised to about 65%. One hatched right on day 18. They rest haven’t pipped or anything. Upon going into lockdown most had
visible air cell upon candling but some eggs were too dark to see into. None are cracked, leaking, or smell. Why would only one hatch? Do the others just need more time?
Any advise?

 

[Cazook]

How late is late?

I've had them hatch as late as day 20.

 

[Farm Lady Fanci]

This evening will be day 21. Not sure what went wrong. The one chick that did hatch is perfectly health and was right on time. The eggs came from Craigslist, so maybe they weren’t the best. Idk.

 

[cloudstreaker]

I have the same issue. Our recent hatch out of 40, 9 hatched and they hatched on day 21! I have no advice but wanted to let you know you're not alone with this issue.