How to properly hunt waterfowl?

So I recently got Salzwiesen Park, and the duck and cover DLC so hopefully I can get some trophy waterfowl. Ive set up 4 different locations for decoys and I’ve killed many hundreds of ducks, and spotted even more, but so far absolutely nothing. I’ve probably hunted for waterfowl for like 8 hours now and all I got to show for it is 3 level 2 mallards that had diamond potential. Surely I’m doing something wrong? Please help this is starting to get frustrating