They go through periods of rest, when they sleep and are very still and quiet. Pipping, both internally and (especially) externally, takes a lot of energy out of the chick, so after pipping they usually rest and sleep for a while, and may appear still and quiet. Day 20 is still early, give it time and resist messing with it. If day 21 has passed and you still don't see an external pip yet, you can candle the egg and see if you can notice an internal pip (the shape of the beak sticking into the air sac) or check for movement.
