There are many factors which affect egg hatch rates. Perhaps not many of them are fertile due to a problem with one or both of the parents? Or perhaps your duck isn't a good sitter? A good sitter will leave the eggs to stretch her legs, defecate, drink and eat, but it won't be for long, a bad or inexperienced one will let the eggs go cold before she returns to the nest and this may cause them to cease developing, or will fail to care for the clutch properly (some birds just don't have the right instincts to get them to, say, turn the eggs, or sit on them long enough).

It could even be the conditions of her nest - temperature, humidity, the ability for the mother to turn the eggs and move them around to keep the temperature stable - perhaps her nestbox isn't in the right place or isn't designed well enough?

My advice would be to hatch them under your hen if that's what works. If your hen and duck go broody at the same time you could try hatching the eggs under the hen, taking the ducklings (preferably leaving the hen with some of her own real chicks to minimise stress) and putting them under the duck in the night - this can fool a broody animal into thinking the new young have hatched from her eggs.

Though be aware that this may not always work, so keep an eye out for signs of mother rejecting the ducklings so you can rescue them and pop them into a brooder.

Domestic Ducks aren't good mothers or egg sitters. I know a lot of hatchers who say that and I agree with th em. Many people get the duck eggs and give it to a broody hen or hatch it themselves. Also, do you have the proper size of the nesting box? I would just give it to the hen. Inexpeirienced ducks don't know what to do with the eggs sometimes.

she probably dosent sit on them long enough

This is the second time my Rouen duck has sat on her eggs. She never hatches them out . They always end up going bad. What's wrong? We put her one of her eggs under a broody chicken and she hatched it out fine. :/