If there's been any sudden drop in humidity it can cause the membrane inside to shrivel up and wrap the chick up (like a plastic bag over one's face). I've had it happen a few times to me. I've been 24 days in and finally have to remove all the other chicks and when I went to toss the remainders I heard faint tapping on this inside (that's a sure way to tell if the chick is still alive). So I quickly replaced it in the incubator but it had been too long, it never pipped and after 12 more hours I thought I better help it. I gently cracked open the top where the air cell is and the membrane was solid and dry like paper-no wonder it couldn't get through it. The chick was very weak from trying so hard and lack of oxygen so I ended up hatching it totally myself. It did ok. After a few days I couldn't even tell which chick it was anymore. But, had I not left it out of the incubator for so long, the membrane wouldn't have dried and it may have hatched perfectly fine on its own. I just felt I should help it since it was my fault for opening the thing in the first place. It's so hard to just wait and it's really too bad there's no other way to see what's going on inside that egg without opening the darn incubator!