As much as I hate to believe it, sometimes it just ain't gonna work.
Bugs me to no end while goose hunting. we may get well over half of all flying flocks to at least work us, and limit out, but that doesn't do it for me. "Why didn't they ALL work us?"
Still haven't found THAT answer.
OH, your question: I think that improper concealment is often the #1 reason for not killing birds.