I've only had 4 batches of eggs hatch. The first were 3 under a broody, all broke through the air sac, and only 2 hatched. I couldn't understand why the 3rd didn't make it. Then, with 24 in an incubator, only 4 survived hatching, many pipped but didnt make it out. I chalked this up to the temps being just slightly cool the whole time, as they hatched the 23rd day. In hindsight, I wish I'd helped the 2 others, but didn't. Then, of 5 more under a broody, all were happily peeping in the shells, but only 3 made it out. That left 5 more under a broody a few days later....and after watching nature take it's course and losing so many babies (plus, 3 hens playing musical nest on these eggs, knocking 2 out in the process, that I managed to discover while they were still slightly warm) I took charge.They'd all broken the air sac over 24 hours prior and 2 had pipped through the shell and then no progress, I took a tiny tool and cracked around each shell, the equivalent of them zipping. I gave them a few more hours...but when it was time for bed and still no progress, I took it upon myself to pop out all 5 of them. A few were completely lathargic. A few were a little bloodier than I'd anticipated, but I did it anyway (I KNOW this is a no-no....see blood=stop...but I didn't). I put them back under the hens that were sharing the nest, and went to bed, expecting the worst. I'm not advocating this for everyone, but all 5 babies that I'd totally intervened with are happy little chicks. And I'm pretty confident they wouldn't have made it otherwise. I think my biggest push to do this was the fact that we are in the city, and the dad to these babies had to go to a country home, so they were my last shot to have some babies but I'm definitely glad I did it.