A decoy is the last piece of equipment that is necassary to call and kill predators. If you are hunting areas that you would see a coyote by the time it would be able to see a decoy, and you're NOT seeing coyotes, than a decoy won't help one bit.
And you don't have to be hidden away very well either. We sit in open country all the time and kill coyotes. I wonder if you're getting too far back into cover where you're not able to see your surroundings well enough. You need to sit where you have a good visual, especially your downwind side. Best if the down wind side of you is open ground. They can smell a human from a couplel hundred yards easily.
Don't over think the sounds you need to use. Simple distress still calls more coyotes than any other sound that you can buy.