You can't call coyotes in an place that doesn't have any. (it you don't see any "Fresh" sign, and you can't get them to howl, your time may be better spent calling other spots)
They have to hear the sound to be able to respond. Some places are very windy and the sound just doesn't travel. There have been times where I did the drop and roll deal with a spotted coyote and 300 to 400 yrds away, I couldn't hear my buddy calling to this coyote at all. I understand that I don't have anywhere near the hearing a coyote does, but if at 400yrds I can't hear it, it's likely that a coyote doesn't hear it at 800 yrds, which means you have to do tighter stands in area's they are going to hold up in.
Also,
I have terrrible results calling where I see scat. (scat in the road, I would try doing a stand, lots of howls no shows) so I started walking in a few hundred yards from where i see the scat and calling.. Much better results.. I even called the east side of the road.. called in a double, heard a pair howling on the west side. I shot one, fromt the double, then hauled the dead one to the truck, got a drink, then walked out on the west side and called in the other pair, and shot one. I shot two coyotes within 800 yrds of each other, with the truck in the middle, just doing this trick.
I also found that you get better results when you hide your truck, I have a car cover made from cammo that i made.. It's easy to pull over and hides the whole truck.
It wouldn't surprise me if you said that you sit facing into the wind. i know there are differing opinions on this one, but if you agree that coyotes hunt using thier nose, then sitting with the wind in your face and the call out in front of you means that you are getting busted by scent and may not realize it more then you know.
If I have to sit facing into the wind, I put the call off to the side and upwind of me, You would be surpised how many times i see a coyote coming in from the down wind, run right past me, get close enough to wind the call, and turn around to run only to get a dose of shotgun pelets at 15- 20 yrds.
Any one of these thing, plus you movement on stand could be the issue.