Have you ever Incubated duck eggs with chickens eggs? From what I understand (bc I haven't actually done duck eggs yet) the duck eggs require very high humidity throughout the incubation period, much higher then chicken eggs. I know a lot of breeders that actually mist their duck eggs daily just to keep the humidity high enough to have a decent hatch. That's the reason I ask if you've ever done them together before. Wondering if so, what your hatch rate was on each type?
My next guess would be genetics or nutritional issues. Do you use any kind of breeder supplement? What brand/variety of feed do you use?