Can the eggshell be too hard for a duckling?

[SchCu18]

Yesterday day 27 three duck eggs have little pip holes, today day 28, more than 12 hours later they have made no progress. I can hear them chirping and pecking at the eggshell but nothing is happening. How long do they usually take to hatch? What should I do?

 

[Pyxis]

Ducks are notoriously slow. As long as they're still moving around and peeping I'd leave them be, they probably aren't ready yet.