I have had chicks hatch as long as at day 25. If the temperature drops it may take a day or more longer for the chicks to hatch. I put them in the hatchers usually around day 18 but have put them in on day 20. I have written the wrong date on the calendar and the chicks really hatched on time but in the incubator. If I have one that gets stuck I also mist the shell with warm water. I have had very few that haven't made it. Once in the winter I lost power during the night and when I checked the incubator the temperature was 78. I put the incubator on a power supply (battery with an inverter) and got the temperature back up and to my surprise I still had a very good hatch. I candled and saw movement so I knew some may survive. I also mark my eggs and put an X on the ones that I think may be quitters.