If your humidity is too high during incubating your chick's grow to big in the egg and the air sack at the top isn't big enough for the chick's to break out and use the space to spin there head round to start piping out. Like I've said is previous posts buy a hygrometer off eBay around £15 and keep your humidity at 35-40% for 18 days them for the last 3 days up it to 65-70%. It's works boys I've messed around with all different makes over the years and it works with them all.