i'm sorry i don't have any answers for you. i've never incubated duck eggs. But with chicken eggs, it's not unusual to have them hatch a day or sometimes two days after they're supposed to. Hopefully a duck specialist can give you a better answer.
The hardest thing to do is to not open the incubator the last few days. Since you know that they are still moving boost your humidity and get the brooder ready.
Temp. variations can cause a hatch to be a little late or the day count could be off. the first full day of incubation is day 1. Like if I set eggs at noon one day the next day would be day one.
Seems that different breeds of ducks have different incubation times too so you may need to look up the time for your breed.
I know Im way late in responding... I did get one beautiful baby named Plumpton!! s/he is doing great and you should see how big s/he has grown. I now have another appleyard in the incubator set to hatch around the 8th or so!! Candled and is a movin somethin fierce.