Why aren't my baby ducks not hatching?

i ordered ducks eggs so that i can hatch them. i ordered 10 mallards and 2 buffs. Friday (yesterday) would make them 25 days old in the egg. which i read is the time for mallards to hatch out but they haven't even started hatching. i don't know what to do bc i have hatched chicken before and they hatched on the day they were suppose to. i am concerned. 4 of the eggs when i candle lighted them had black in the bottom of them which means dead embryo. the others look like there is a duck in them full grown but i don't know why they are not hatching?? please help me!