Please don't get brewers yeast it doesn't have near the niacin duckling need, If you have a TSC get liquid B complex, it is wonderful for getting the right amount of niacin into them quickly you can give tiny drops right along the bill giving it time to swallow each time or tiny drops right inside of the bill giving it time to swallow. Also adding to water which will help them all. If you want to add a boost to their feed get Nutritional yeast. Not baking yeast and add 1 Tab per cup of feed. But since this little one isn't eating you need to give the liquid B complex. Have you tried a warm soupy mix for this duckling?
This could be something genetic or with incubation but trying the liquid b complex is always a safe inexpensive way to try and help.