Question about Duck eggs/hatching

[Tinydinomomma]

I have been raising ducks for about 3 years. I have 11 females that are laying age. From late March to early June between a few girls sitting and me incubating we hatched 64 healthy ducklings ( I lost count of the amount of eggs)...but they are still steady laying. I've continued putting them in the incubator but the hatchings have been zero. They eggs either quit developing, or they actually explode. Nothing new with the incubator, the settings are correct, I have not changed anything..but I've also never had them continue to lay this late in the year and continue to want to sit the eggs. They are mallards. The oldest is 3 years old, the youngest is a year old. Can anyone shed any light on why the eggs are not continuing to develop to full term (for lack of better term)? Just trying to figure out why all of a sudden, my eggs are not hatching.
Thanks.