Does she get grass and bugs? If the crop is the issue for sure I’d wonder if she’s getting grit like corn.
Isolate the bird if it’s having problems, 1, to protect and recover 2, in case is contagious. I’ve never had this problem, but yeah, are you giving it any grit like the other replies...
They need the humidity. I’ve never heard of dry incubating, but it doesn’t sound right to me. I’ve hatched 100s of ducks. Most eggs are supposed to lose about 14% of their total weight in water during their incubation. So get the
on a .1gm scale, weigh them. depending on what type of duck...