Nutridrench is nice I have that and also Rooster Booster Poultry cell is another excellent supplement. You could be right perhaps the vitamin D will help her. Definitely worth trying!
Its funny I have the opposite problem here ... mostly ducks and just a couple chickens. They share the all flock but for some weird reason my chickens love to eat the floating waterfowl food too . and they look so silly pecking it off the top of the water.
As for the niacin, many ducks do not...
May I ask why are you feeding layer feed? For ducks an all flock feed or a waterfowl feed is better - especially if you have any males (drakes, roosters- the calcium is bad for them)
ducks need nutrients that are not included in layer feed thats formulated for chickens.
Especially the extra...
Liquid calcium gluconate 1 ml orally with syringe no needle.
you can give for a few days up to a week
https://www.backyardchickens.com/articles/safely-administering-oral-medications-to-all-poultry-and-waterfowl.73335/