It's my (probably unfounded, but carefully formulated, lol) belief that most late-hatch deaths are due to failure of the air cell to develop adequately. Have you been candling? What does the air cell look like at hatch? Do you live in a humid area? What humidity are you running the incubator prior to lockdown?
Because I live at sea level in a humid part of the world (SE U.S.), I run my incubator humidity as low as it will go in the summer, then bump it up at hatch time. This allows the air cells to develop fully in time to hatch. They should take up between 1/4 and 1/3 of the volume of the egg at hatch time. Less than that, and the baby will try to pip into the air cell and will either be unable to (because there's not enough room), or will do it and then drown (because fluid flows up into the small space).
Sorry you've been having so much trouble. I know that's terribly disappointing. Unfortunately, most recommendations are to run the humidity higher for ducks, and in fact the opposite is often true, though local conditions and your specific incubator will have a strong impact on the right humidity for you.
Good luck with whatever you decide. Ducklings are adorable, and I love those I've purchased as much as those I've hatched, so if you're ready to give up on hatching, you can still enjoy ducks.