Ducks like to take their time, so don’t worry too much. What you saw there was an internal pip, meaning they have broken into the air cell and are now breathing. Within the next 12 hours they should externally pip, meaning they break the eggshell. After pipping, they will zip. Zipping is like pipping but all the way around the egg. Within the next 12 hours after externally pipping, they should hatch completely. Sorry if I’m over simplifying this but that’s the best I can explain it. I hope it helps.