In my experience a bird will only sit on a clutch of eggs when it feels ready - in other words, when it feels it has laid the number it wants to.
Sometimes birds will never sit on eggs because broodiness has been bred out of them - for example, hybrid chickens that are bred to lay large numbers of eggs.
If you are having trouble getting your duck to sit even after she has laid a large clutch of eggs, you could try getting a bantam hen. They are extremely good mothers. She may only be able to hatch out 4 to 6 eggs depending on how big they are - but that is probably enough.
Sometimes birds will never sit on eggs because broodiness has been bred out of them - for example, hybrid chickens that are bred to lay large numbers of eggs.
If you are having trouble getting your duck to sit even after she has laid a large clutch of eggs, you could try getting a bantam hen. They are extremely good mothers. She may only be able to hatch out 4 to 6 eggs depending on how big they are - but that is probably enough.