Yes niacin deficiency comes on quickly as it is a water soluble vitamin that the body cannot store. My son has pekin females that are laying -- he was feeding them organic layer pellets designed for chickens, in the belief that adult pekins don't need extra niacin. One of his girls got confused overnight and could not find her way out of the coop -- not even following the others that periodically came to get her in response to her frantic calls. I started them all on brewer's yeast and within 5 days she was back to normal. My son then failed to repurchase brewer's yeast and within a couple of weeks that little duck was confused again. I now buy the brewer's yeast and make sure they all get it everyday!!
problems with gait -- incoordination of the legs, falling, aand being unable to walk are common symptoms of niacin deficiency. Other neurological issues [such as confusion!] occur too.
Others have given you advice on providing niacin now -- both urgently in the form of VIt B liquid or capsules. But you also need to start your pekins on Brewer's yeast [check the lable to make sure the brand you buy contains Niacin/Vit B3] and feed your ducks 1 tablspoon brewers yeast sprinkled on each cupful of duck food. They will need this for life!