duck disease? deficiency?

Hey all! I got six little ducklings about a week ago and they were all fine and growing until yesterday we noticed one was pigeon toed and stumbling, and another today. They also seem to be rocking their heads back and forth. I thought maybe it was a niacin deficiency, but have been adding nutritional yeast to their food since I got them, and also I haven't found anything in my research about this head movement being related to niacin. These two stopped eating and drinking, so I've been trying to force feed them nutritional yeast and crushed mealworms in water in a dropper. I'm going to try to attach a video, they've been doing this with their heads for hours. Would love any advice or input!

oh it won't let me attach a video?