Duck eggs don't all hatch in 28 days. Some take as long as 35 days. Check your breed incubation time.
The other potential issue is if the temp is low. It doesn't take a lot but say a degree low over time of incubation will extend hatch time by two days and lower probability of hatch. You can use multiple thermometers and pull your hair out as where you place them in incubator will read different or use a digital oral thermometer and trust that to be spot on.
You didn't mention type of incubator. If the incubator does not have a fan then the air is very layered by temp. 101.5-102F is incubation temp used measured at top level of eggs.
All you can do is sit back and wait it out. It will hatch or not and troubleshoot from all the data after hatch to determine what if anything was wrong and change that one thing next go round. The more you hatch with same equipment the better your results as you fine tune techniques for your incubator, thermometer, hygrometer, manual or auto turner, etc.
