Sorry, I'm only now seeing that I got tagged in this thread.
It sounds like she having some problems, but I'm not too sure this would be niacin related, and I say that for two reasons, her age, and the onset of the problems. For the most part, you're most likely to see a duck showing niacin deficient problems when they're young around the three-week mark, as they age they can develop problems but its unlikely unless the diet has been changed significantly. We also have to consider the onset of her problems and realize a deficiency is most likely not going to occur over a day or two, that's not to say you didn't notice the symptoms at first, however.
But to answer your question, it is very common to only see one bird affected by a niacin deficient diet, the contributing factors haven't been studied much, but the general consensus is that it is most related to genetics, and how each individual bird converts niacin from tryptophan.
For more info, you can read the bottom of this post,
https://metzerfarms.blogspot.com/2011/03/are-my-ducklings-leg-problems-due-to.html
If you do believe something is wrong with her that needs to be addressed, your best bet would be to take her to an avian vet in your area.
If not, we can help the best we can, and we'll need to start off with a nice anamnesis, and we'll need you do do a quick physical examination.
Would you be able to answer the following questions?
How old is Rhonda?
What is her setup like, do you have any pictures of what you keep her in?
Where did you acquire her?
What is she being fed?
Have you had any problems with her in the past, any recent scares?
Has she eaten anything unusual recently?
Does she get good exercise and water bathing time each day?
Could you a picture of her feces next time she poops?
Would you be able to bring her inside the house and do a thorough physical examination on her?
Check both eyes for signs of redness, inflammation. Also, perform a menace reaction which consists of moving your hand slowly towards her eye, there should be a reaction of sorts.
Check both ears for signs of infection/gunk (ears are a small hole behind the eye)
Check her mouth for any sores, abrasions, etc ( A flashlight can be used if needed)
Check under her feathers for any sign of ectoparasites, a flashlight can be used if needed.
Feel both of her legs for swelling or warm spots, also lookout for signs of Bumblefoot on her footpads, and swollen hocks.
Lastly check around her vent area for signs of vent gleet, flies, etc.
