With having them on an All Flock feed, having been given Nutri Drench, and having them also on Life-Lytes, I'm not sure if this would be a niacin issue. There are a couple diseases that can cause this too, like toxoplasmosis, which the birds can pick up from cats and also can be passed from a mother duck to the babies right through the egg, although rarely, and Riemerella anatipestifer, which is more likely. It's a disease that attacks the nervous system and causes shaking and loss of balance. A key symptom is ducklings falling over onto their backs and paddling their feet in the air. And it's also contagious, so it is possible that if it's one of these diseases, it could be passed to the other ducks. It wouldn't hurt to put this one in quarantine until we figure out what's causing the trouble.