How old are your hens? If they are only a year or two then they might not yet be getting the message... If They are 8 years or older they might just be too old... You'll know when they go broody when they don't get off their nest even late in the day and start to have a little "spazz attack" when you go near them. If they don't go broody and you really want ducklings then I suggest an incubator? The cheap styrofoam ones I mean.