It took a few days for me to put it all together. One duckling keeps stepping on its own feet and this makes it hard for her to walk... but she has stopped gaining much weight for a couple of weeks, and won't stand straight. She stays hunched over (runner duck but stands like a mallard). Another walks funny, but his appetite is good. Yet he finds walking harder and harder. The third, the biggest duckling, has not kept up his weight gain and now doesn't want to stand up. All of a sudden I am thinking niacin deficiency. Yes, I add yeast to the chick starter regularly. But they have moved from crumbles to pellets and I wonder if all the yeast isn't sinking to the bottom. Only certain ducks are willing to eat the "dust" at the bottom, so some are getting more yeast than others. My ducklings are seven weeks old. I plan to feed yeast individually for a few days and switch to an all-flock food that says ducks and broiler chicks. Any other suggestions? Help! It's my favorite duck that's down...