What are they being fed?
Often, it is a niacin deficiency, and it is different for each duck . . . . some manage okay with lower niacin intake, others develop leg and nervous system problems.
You can get some niacin capsules, just plain niacin, not the no-flush or specialty kind. Put between 100 to 150 mg of niacin in a gallon of water, and make that available to the duckling. It should not harm the other ducklings.
Storey's Guide (if I recall correctly) states that ducklings need three times the niacin that chicks do.
Another option is to sprinkle a tablespoon of brewer's yeast on their food.
Time in a lukewarm tub of water deep enough to float in, with constant supervision, a few times a day may help muscles, too.
