After internal pip (that's when you first hear them) they can rest for a full 24 hours before they have the energy for external pip and zip. During that rest period, a lot is going on. They are absorbing the remaining yolk, and blood in the membrane vessels. They are also working on their breathing. When the CO2 level in the egg gets high, and the O2 level gets low, her O2 level goes low. This causes an alteration in her movement patterns. Her legs go from a reciprocal left/right movement (walking or swimming) to a synchronized thrust (jumping or pushing) Without this change in pattern which is caused by the O2 level shift, hatching will not succeed. Worry not. After she externally pips, she will then rest for quite a while before zipping. If I have a chick who is distressed and not progressing past the external pip stage, I often wait until the end of the hatch before assisting. As long as she can still breathe in the egg after that external pip, it won't hurt her a bit to sit tight for a full day. This approach has saved me from assisting before the chick is ready to hatch.