PICTURES!
What do you feed her/him? (Brand, "style", etc)
Ducks, particularly young ducks, are sensitive to diet in ways that tend to appear most prominently in the joints.
We want to see both knees, for comparison.
Also, feet! Have you looked for foot injuries which might promote inflamation at the knee?
Lay off the peas. Peas are HIGHLY variable in their nutritional aspects, but can contain high lelvels of trypsin inhibitors, tannins, and lectins - all of which can interfere with a commercially complete diet. Without knowing what peas you've got, better to eliminate that variable first, consider working it back in (10% or less, by weight) later, after the bird has four or five months, and the majority of its development/growth behind it.
A lack of niacin is quite common in young duck diets. Suppliment - nutritional yeast is a common, off the shelf, solution. Young ducks need niacin like humans need niacin, at levels well above what a chicken needs. If the feed isn't specifically formulated for waterfowl, or one of the big name feeds whose niacin content has been previously verified as adequate, that's a potential you want to chase down fast.