It can be a nutrient deficiency maybe reproduction issues. Sometimes even a duck that is eating the best feed it can still have problems some how not getting all they need out of the feed. I am not a vet but I will suggest this that you start her on Calcium citrate and Liquid b complex Calcium citrate can be found at most pharmacies ( Walmart) and the liquid b complex is at TSC most likely in the cattle section it will say injectable but we use it orally 1 ml 2x a day over a yummy treat and 50 Mgs of calcium citrate per lb of body weight this may or may not help but we won’t know till you try. Def worth trying for at least a week.