baby ducks wobbles

About a month ago our ducklings hatched and until this point they all looked pretty healthy and lively. However since a week ago, I noticed this one duckling looking slightly smaller than the rest and more lethargic. She still eats as far as I can tell, haven't seen her drink in front of me...

I got these ducks just over a week ago and they were both fine until Saturday. One started getting off balance, then by the end of the night she kept falling. I have been giving her nutritional yeast and some niacine since Saturday. She doesn’t seem to be getting better. Does niacine deficiency...