Like Hans said, make sure they have access to water. You can also soften their
feed with water. Some feeds can even be sprouted or fermented (
http://www.scratchandpeck.com/wp-content/uploads/Fermenting-Feed_7-27-2015.pdf), which helps add niacin and other nutrients. (If you're feeding a
chicken feed, make sure to add niacin or brewer's yeast to their feed, as
chicken feed has less niacin than ducks need.)
It could also be that the ducks are not used to eating feed. You can try chopping up grass and
dandelions and other greens to their feed, as that might look more like food for them, and it will add more nutrients into their diet. If you add in the greens, though, make sure to supply grit if you're not already, to help them digest it (if you can't find grit, scoop up some sandy/silty dirt and put it in a bowl--I did that for my ducks and it worked fine).
I hope that helps! It can be so overwhelming when you first get ducks, especially if you weren't planning on having any for a few years!