I think ducks, like people, can have off days. Even my pup occasionally refuses her breakfast [but thenneeds feeding early afternoon]. I wouldn't worry about occasional, non-specific, ducky down days.
My son feeds his adult female pekins brewers yeast with all their feed -- he [or I] sprinkle 1 tablespoon brewers yeast for each cup of feed as a protective supplement. He did stop it once and one of his females got confused -- she recovered quickly when the brewers yeast was restarted.
You...