I agree & disagree with the above comments. I COULD be a molt thing and they will grow out of it but, I hate to be the bearer of bad news, sometimes they just have a bad oil gland that will not get better.
I had a duck this last year who's oil gland was clogged/ and or broken. I gave her soaks in epsom salt, rubbed her gland with oil (yes, she hated that and it made her extremely dirty as the dirt now stuck to the oil) massaged her and did anything I thought would help for several weeks.
I finally ended up taking her to a vet. I have never taken a "barnyard" animal to the vet, but I did so love this duck. In addition to the oil gland problem she had a bad bumble. In the end there was nothing I could do for her short of making her a house duck. There was no way she would survive the winter without the ability to water/ weather proof herself.
I wish you the best of luck with your flock & hope it is something that will correct itself.