@bekkanblue
My lockdown is tomorrow, also. I've just got two eggs going into lockdown and both are iffy shipped eggs with wonky air cells.
Candling is your best way to determine if you have quitters at lockdown. If you have viable veins that you can see (look near air cell) or you can see movement, then don't discard. If you're just not sure, go ahead and leave them in there. At lockdown your egg (well, the part that's not air cell) is going to look very dark because it's mainly chick. There may be a bit of clear fluid up near the top of the egg as the chick still has a couple days of growth left. Be suspicious if you have lots of clear fluid at the small end of the egg.
If in doubt, leave the egg in. Don't sweat lockdown...either they'll hatch or they won't. Remember too that chicks will often take 24 hours from that first external pip to a hatching zip. During this time they're often absorbing the last of their yolk and the blood-filled veins in the membranes are receding, so do not -- repeat DO NOT -- go in and help or you'll have a big mess on your hands and a compromised chick.
Also, don't sweat if your eggs don't hatch exactly on Day 21. Incubator temps can sway a day either way and some breeds are just plain pokey. I've never had a Marans hatch on Day 21, more like Day 22/23 and I've even had a Welsummer hang out until late Day 24 still alive in the shell.
Good luck and happy hatching!