I have had 3 ducklings, I received in very bad condition. They were not fed enough, had bumble feet and were missing their down feathers. They were 3 times smaller than my duckling.
All that made them extremely greedy and they ate everything they could get their bills in. They started growing very fast and gain quite some weight in no-time. I was certain I was not giving them enough niacin, and fed them yeast daily. Later on, I have figured out they were no longer deficient, the only issue they had was their weight. Their muscles could not develop fast enough for them to move the same way as they normally would. At one point, they stopped eating slugs (they became too large and had other food they preferred when foraging) and I only fed them oatmeal. I noticed slight improvement and put the remaining duckling on a diet. He fully recovered and reached his adult size. After that he could eat whatever he wanted and had no issues with legs whatsoever. He was slightly obese and fantastic eater - he would come and eat bugs and worms literally from your feet. He was excellent forager.
If your ducks are growing fast or gaining a lot of weight, that could be the problem. Niacin in form of yeast will not harm them, so you can add it as a precaution. Better safe than sorry.
