Hard to say why you haven't had any success yet without seeing it first hand. I don't presume to know the thickness of the vegetation and cover, or the type of terrain where you are hunting.
Possibilities:
The coyotes are not within range to hear your call at the time you are there.
The coyotes that are there can hear you, but have been well educated to the rabbit sound, and will avoid it. Maybe try a fawn bawl, bird or kitten distress, coyote distress, coyotes howling, ect.
Their bellies are already full of some other type of prey.
You have called in some but haven't seen them..............because they smelled you or your "footprints" into the area. Try to set where they have to expose themselves in order to get downwind of the caller. Try to set up where you can see downwind to some degree too.
You have called some to the edge of cover, but they saw a reflection or glare that wasn't natural in the vicinity of the sound, or they could see your vehicle back behind the vicinity of the sound...............they will stay in the cover.
Your volumn may be too loud, or too quiet, hard to say without being there.
You may not be staying there long enough for them to come all the way in before you get up and leave (happens in open country where they may be coming from over 1/2 a mile)........Stay put for at least 15 min. 30 is better in open country.
As you can see, there are TONS of reasons why we may not get a coyote.
If you think that the coyotes are not there when you are, it doesn't hurt to try the same spot on consecutive days. Just don't overdue it and educate them to every sound in your arsenal. I've always liked to give the area a little rest, no matter if I had any success in that spot or not.
When playing the Electronic Caller, are you playing it constant or are you playing for a bit then pausing (silent) for a bit?? Which do think is more realistic??
You can try to locate some before calling (by listening for their howls early morning or using a "coyote locator" sound on your Johny Stewart). A person can hear coyotes howling on a calm day out to about 2 miles in open country, less in timber/heavy vegetation or hilly country. Once you've got an aproximate fix on their direction, you can close the distance quietly and set up/call every 1/4 mile or so. Once you learn to estimate their distance from you by the volumn of their howls, you can sneak right into position and go for glory. If the country is "open" enough, you can locate them via binoculars/spotting scope (works really well when everything is white). Once you spot some, sneak into them (try to get 1/4 to 1/2 mile from them without exposing yourself) and then go for glory.
Your best bet may be to locate someone in your area that is having or has had good success, then go out hunting together. 4 eyes are better than 2 when it comes to spotting incoming predators sometimes anyway.
Hopefully this gives you some ideas and some better luck.