I think it depends on what temp you are incubating at and maybe even what strain of coturnix. Mine have been hatching at day 18 but online I keep seeing 16-18 days. My last hatch (about 5 days ago lol) they started hatching at 17.5 days and finished on day 18, all of those eggs were from a new source to me and I didn't have any of my own eggs in there. I have a mixed set in lockdown now, part my eggs and part the same source as the previous hatch, they should hatch hopefully within 2 days.Yes when i candled the eggs the air cell was tiny. So they probably drowned or developed early. But Day 18 was very late for a hatch,i thought they usually start pipping around Day 16.