Moses, If you can part the Red Sea, you can kill those coyotes. LOL! Little satire there!
You just have to find their weakness. First of all, I only call a short series every 3 minutes. I think too much calling satisfies their curiousity and allows them to pinpoint you more accurately. What I describe here is the situation in the area that I call while using hand calls. Electronic calling in heavier cover with the caller placed away from you may be an entirely different story.
I use two howling sequences then I break into distress calls. In the situation that you are describing, I would try a light bird distress cry. I also find very few refusals with hand squeeks. Are you sure they are not seeing you moving or catching a scope glare? Both will hang them up just out of rifle range as will continual calling with educated coyotes.
The other thing to consider is what Rich Cronk suggested. You may be calling from the direction that they expect to hear a caller calling from. Try backsiding them at a different time of day (morning vs. evening) and use a different sound.
Also strongly consider the "dog days" where nothing you throw at them will turn them on. Everyone that I know that does any amount of calling will have some refusals. Inevitable refusals are a harsh reality and there are no excuses necessary for this typical passive coyote behavior on certain days. "Dog days" have nothing to do with the sounds you are making and have everything to do with unpredictable, passive, "dog day", "lift their leg and go" coyote behavior.
I had a coyote hold up on me 2 days ago at about 200 yards. I was just getting ready to squeeze off when the coyote looked back over his shoulder. I fully expected another coyote to show up. The wind was blowing to the S/SE and this coyote was S/SW. About that time the wind switched to the S/SW and goodbye coyote. All I could do was cuss at the "downwind boys". LOL! Just when I start to question myself, reality slaps me in the face again.
Utah C, maybe I just stink real bad! LOL! Wiley E