Sorry I don’t have a lot of personal experience with this problem. But I can still try to give some insight. What feed are you giving them? Layer? Do you give them free access to feed. Maybe you could start measuring out feed daily? Also are they free range. That would make sure they are staying active and running around and stuff. Also a lot of feeds include cracked corn to keep them with a little fat but you could look for a corn free feed. Not sure if that would actually help a lot though. I just know corn is like the number one fattening grain for my birds.
I’m sorry your having this problem

. Unfortunately Pekins are just very susepctable to it because they are technically meat birds. Next time you might want to look for intentional layer pekins or another breed of ducks to try and avoid this. I hope some part of this helps! Good luck!!