Also. How old are your ducks?
Older ducks lay eggs with more porous shells, and are harder to hatch.
It isn’t impossible, but it is a factor that may come into play to solve this puzzle-
It’s late, so I apologize for not looking up the conversion for Celsius to Fahrenheit
But
I’ve had reasonable success with my Welsh harlequin duck- and turkey - eggs this year, in a HovaBator incubator and a second HovaBator for hatching.
(Vertical) auto turner, forced air heat, and a humidikit...