I am so sorry for your loss, do you want to explain details like how old they were what you fed them where you kept them, all these things may help us help you figure out what happened.
I know exactly how you are feeling, I looked everywhere for the chick except the water bucket when that finally dawned on me it was too late. These things happen and mamas bounce back and contniue to take care of the ones left whether human or ducks or chickens. We just dwell on it longer.
I'm so sorry for all ya'll that have lost ducklings. Thank goodness I haven't lost a duckling yet but did lose my first chick to drowning a couple weeks ago. Broke my heart.