What do their air cells look like around Day 20? My experience has been that when I run the incubator at the recommended humidity levels, the air cells are underdeveloped and the ducks drown after pipping internally. They need a large air cell, about 1/3 the volume of the egg, so they don't have to hold their bill up so high in the egg after pipping internally.
If it were me, I'd try running a batch with super low humidity--just don't add water at all. Misting the eggs will have a similar effect--I know it sounds counter-intuitive, but misting actually helps moisture to evaporate out of the shell--it has to do with polarity, how water is attracted to water, I think.
I don't mist, nor cool--I just run my humidity super low, and my hatches average 80-90%. My most recent hatch was the worst I've had this year (about 63%), and I believe it's because my air cells were underdeveloped.
If the temps are fine for chickens and other poultry, then you're quite right that it's fine. I'm betting the issue here is humidity. Good luck!