The humidity level is critical to a successful hatch.  It needs to be consistent at 45% until 'lockdown' at day 18, when it should be raised to 65%.  Too low and the chicks will have trouble hatching. Too high and there's a real danger they'll drown in the egg, or not 'fluff out' properly after hatch. Getting humidity levels right is the hardest part of incubating, in my experience. It's the only time I've ever lost a chick after hatching – the humidity levels were too high. Despite hatching without a problem, she wasn't able to dry out and died from cold before I was able to get to her.