With eggs that have pipped, they generally need help if it's been about 18-24 hours with no progress. When helping them, watch out for blood vessels and don't let them come out of the egg until their yolk is absorbed -if it isn't, you'll be able to uncurl their head and see it in there by their legs, shining a light in helps. Also if they're trying to "zip" the egg and don't seem to be making progress that's a problem, especially if you can't see the beak anymore -I just had one die that way last time I hatched.
Eggs that have not pipped internally with the others, and especially those that haven't pipped by the time the others have hatched, are the ones that are probably malpositioned and will suffocate if they aren't helped. Another of my eggs died from this last hatch, because I was at school while it was trying to hatch.
Most people say that you can't help an egg that has not internally pipped on its own, however I did just that when I saw that one of my eggs had failed to do so. The other eggs had already hatched, and this one was just sitting there. I could see a little movement when I candled it, but no beak and no cracks anywhere, so I made a hole in the top of the air cell to see what was going on. I couldn't see the beak at all, just the side of the chick, and the blood vessels looked weirdly dark -this means low oxygen, they should be a brighter red; something I learned from reading an article on hatching eggs without the shells. She was obviously stuck and running out of oxygen, so I started chipping away the shell, trying to leave the membrane intact as I searched for a beak. I ended up taking off half of the shell before I found it, and causing a little bleeding too, but when I did I was rewarded with the chick peeping at me and uncurling somewhat. She was pretty weak, so I made her a little bed in the incubator out of a sponge [that's how she got her name: Sponge] and she stayed there with her remaining bit of shell for about a day as she absorbed her yolk and got stronger, as when I got her out she was so weak she couldn't even open her eyes. She also had a leg issue and would fall over on her back any time she tried to walk, but after a few more days of feeding her mostly egg yolks with a spoon -that's good for helping the weak babies, I've found- she was a lot stronger and could only be told apart from the others because she was a little smaller.. She's nearly a fully grown hen now.
She's the reason I tell people to check their eggs around hatch time, especially if it's the last egg or few eggs and isn't doing much of anything, because if I hadn't she would have died.
I hope your eggs don't need any of this, though. Most don't. Good luck with them, and I hope you have several fluffy baby birds in a few days.
