If it were me, first I would not worry too much at this point. As I wrote, it is not uncommon. But there may be something missing in their diet. In my experience, some ducks simply have more need for some things than others. So this food - which many many people use - may just be lower in something that your ducks need.
So, I'd get some poultry vitamins - I think they are $4 for a four ounce packet - that can last for months. Give that once a week in their drinking water.
I might add a couple of tablespoons per day of good quality cat kibble to their food, for a few weeks. That can provide some extra protein and vitamins and good fats, perhaps, to their bodies. I tend to think of cat kibble as helpful for feathers and bills.
Otherwise, they seem healthy, yes? The bills are not soft, or have odd bumps or depressions?