I dont know and thats whats frustrating. We have 3 Rouen (2 hens 1 drake), 2 Peking (1 hen 1 drake) & 2 Khaki Campbell's (both drakes) and are now 7 months old. They get feed, kitchen / table scraps, fresh fruits & veggies as well as free ranged all day up until last Saturday when a fox was just feet from the run. They started laying at 5 - 5 1/2 months.
The first batch I tried was 36 eggs only because no one wanted to sit. I knew a few were older than 10 days but hoped for something. 5 ended up cracking a few I saw movement but none hatched. I had the temp at 101-102 and humidity was a bit harder to control because air humidity was 80-100% but in the incubator I was able to keep it more around 40-45%. Air cells all appeared to develop normal.
This batch was 34, 18 from our ducks and 16 from a friend. I kept the temp 100-101 and humidity 35-45%. Of those 14 were duds, 13 I am sure were quitters, 3 looked like they were developing but no movement and 4 I saw movement up until when I candled Monday. One looked like it had an internal pip with its foot (pictured earlier) and the shell turned dark. Tonight at 10pm starts day 28 so maybe something will happen. And their air cells look good too.