I'm curious how a chick would possibly have such a deficiency just out of the egg when it's still absorbing the yolk? You would think it wouldn't happen so fast... Especially since my other 3 barred rock chicks from the same person all seem totally healthy. I'm sure it's possible, just don't think it would be likely.

But then again this is only my second hatching so I don't have a ton of experience. I'm currently waiting on 4 more eggs that were on day 21 yesterday... Not sure how long I should wait, I see no signs of pipping or movement..
It's often passed from a deficient hen based on her diet. Chicks, once hatched, have already absorbed all of the yolk that they are going to absorb, so that process is over.
Not all chickens come out of the shell with everything they need, even if they've absorbed all the yolk sac, any more than every child has everything it needs when born. Things go wrong for a slew of different reasons. Some just need a little help and they're fine. It's FAR MORE LIKELY than you know.
We have raised Silkies each spring from one father and two mothers over the last several years. Out of 20-30 chicks each spring, it's not uncommon for one or two to have a little trouble that the others didn't, even under normal hatching conditions. What do we do first? Treat them with NUTRIENTS. Some just need that extra boost of Vitamins for a day or two, and all is usually fine.
Why usually? We did lose one chick a couple years back that just did not respond to treatment, but that was the ONLY one we've lost.
As for your other eggs, I can't say how long to wait, maybe to day 23. It depends on too many variables that I'm not privy to: quality of the eggs, how they've been handled, the temperatures of your incubator(s), humidity, and on and on and on. We usually give eggs put into lockdown on the 18th day about 4-1/2 days to hatch. During that time, we monitor the eggs for activity inside the shell ahead of the first egg pipping, but once that pip happens the lid really needs to stay closed so you don't dry out the membranes of the hatching chick(s).