I can try to help. Here's the basic process of the hatch. First the chick pips the internal membrane so that it's beak is in the air sac. It is now breathing air, so it can peep which they usually do, after they internally pip, they rest for a while. Once the air sac runs out of air, the chick must externally pip. You see the cracks of the external pip. It can once again breathe. It will then usually rest again. Then the chick will turn around the shell chipping away as it goes, this is called zipping and will appear as a circle of cracks around the shell. The chick may rest again afterwards before pushing the 2 halves apart and hatching, it's a very long process that can take 48 hours. Also, throughout it's struggle it absorbs the remainder of it's yolk sac and absorbs the blood from it's outer vessels As the chicks move around in their shells trying to get into "hatching position" the eggs rock, it does not mean that they are ready to hatch. Even if one makes and external pip, it may not be ready to hatch. They need to struggle to absorb their yolk sac, and if they don't they may die outside of their shell. So don't go in and help thinking that they are stuck. I usually see rocking eggs and hear peeps on day 20 and then they begin pipping on day 21, they then complete their hatch late day 21 to day 22, but like bobbi-j said, they can hatch sooner and later depending on tempatures. I've had chicks hatch day 18 because of slightly high tempatures, as well as chicks hatch on days 25-26 because of slightly low tempatures.
