well chick's have been in incubator for 24 days now, 3 have been chirping since yesterday and 1 has only just pipped since 10am this morning. Now nothing chick is just chirping.
From the external pip to when they start unzipping takes around 24 hours. As long as humidity is 60%+ they should be fine and start to unzip when they are ready. It's an important stage because between pipping and unzipping they are absorbing the remains of the yolk and the blood that's been running through the veins in the membrane.