Angus, Your set up is very impotant. If I was you I would go to the place where you think the yotes are. You should set up and call there.Some of the smarter coyotes do try to circle down wind but if they are hungry they will come right in. I once had a coyote come in and stand directly behind me and I didnt know he was there until I got up to leave. I was set up so the wind was at my back. I think you should get a better howler like Duckdog said. This will help you know which way he is coming in (if he decides to respond).
Hope you get a yote soon!