The feeder could be irritating her neck, but you'd probably see that with other ducks if that was the case.
Maybe apply something soothing to her neck or try some vetrycin where the main irritation is. Unless its mites or another pest which you couldn't see.
Sorry if I'm asking 1 million...
In your wobbly duck post, you stated that one was molting. So, it sounded like maybe both were having feather issues.
Is this the duck that got attacked in the neck area?
Is the neck always wet?
How long ago did you change feed?
What type of feeder do the ducks use?
All flock feed is fine for ducks. I feed it to my whole crew. You mentioned she was having this issue prior to the feed transition. If shes molting that could be part of the issue. Do you see pin feathers?
How old is she? She seems to be having multiple issues considering your other post.