They often start pipping on Day 20 but not always. Your chicks should be fine. Try tapping the incubator and listening for and cheeping. Cheeping is a sign that they are going to pip soon! Humidity needs to be at least 65%. If you're having trouble getting to 65% then put in a wet sponge. This helps a lot!
Today is my day 20 as well!! I have 25 eggs in lockdown, 1 hatched, 1 that I assisted and several pips now. I heard chirping last night. I agree with everyone, listen and look carefully and (try to) be patient!!!