In my research I learned that ducks love peas, I have yet to try those with mine tho. They need more niacin than chickens so if you had a chicken food you could add peas to help make up for it or sprinkle brewers yeast in there food, both add niacin.
I saw this was from November, depending...
I Experienced a similar situation two years ago when I mailed ordered hatchery chicks. I learned that stress contributes to pasty butt which contributes to prolapse and two of them went through a very similar situation as what you’ve described. Another stressor would be they're not warm enough...