Make sure they are getting enough niacin. If you are feeding a chick starter feed you will likely need to add additional niacin. If you are feeding a duckling starter you may not need to add anything. Just check the label to be sure. They need 55 mg per kg.
Not having enough niacin as...
Ducks aren't exactly know for being graceful on land. It could just be a case of him being clumsy. For now I would just keep an eye out for other symptoms, like laying down a lot or not wanting to walk.
Are you giving supplemental niacin with their feed?