They can hatch really slowly. The most important thing is to make sure your humidity is high at 60%, and to stop opening the incubator. Every time you open it, it sends a blast of cool, dry air in and that can shrinkwrap the membrane around the duckling, inhibiting it's movement and making it so it can't turn in the egg to zip after pipping. You know that's happened when the pip hole gets bigger and bigger, but there's no progress after 48 hours.
I've learned just not to open the lid at all after lockdown. Bump the humidity, and leave it alone. Sometimes for 4 days!