If your in them but can't see them get near the beds in the mid morning and cow call like your having a tea party with multiple cows go on for a good 5 min. Then cut it off after 2 min, soft cow every so often. No bugles.
Another thing try bugles before light to locate bulls or after dark. They sometimes will talk in the dark and then go quiet during the day.
Don't forget they will come in silent rather than loud and proud.