If there are no pips on day 21. I will take each egg, put it to my ear and gently tap on the shell. More often than not the chick will scratch back. If there is life signs in the egg I will take a very sharp object and poke into the large end of the egg a hole so the chick can get air.
I have noticed in the eggs I hatch, very often if the chick is large then they get tired easily and can suffocate trying to get out when they run out of energy. Poking a hole in the end gives them fresh air access while they get up the energy to continue hatching, yet is doesn't cause them to bleed out by poking the veins should they not have totally absorbed yet.
And yes, I know, this can cause shrink wrapping due to lowering humidity. But at day 21 I'd rather have to help out a few shrink wrapped babies then have a whole hatch of dead chicks because they couldn't get out of the shell.