And a few more questions/stuff to think about... You may already be a mile ahead of me here.
Are they being hunted hard? Hunted coyotes become very call shy and difficult to deal with. Can you hunt them at night? Looks like you are in NC?
What type of call are you using? Electronic or mouth?
What calls are you using? Rabbit, bird, rodent, deer in distress, etc? Try something non-standard. If they are getting hunted the call might not matter though.
If they are smart dogs they will circle well down wind and try to catch your scent. Many times coyotes come in, but they are never seen. Especially in areas that are brushy or heavily timbered.
Not sure when they den in your area, but out here in the west it is right now and for the next several months. They can become more vulnerable during denning season because momma has to eat! If there is an abundant food source, like a major vole infestation and they are getting plenty to eat, it is likely they will not take risks and stay or become nocturnal. And once again, you are kinda screwed
I have lived your frustration! Sometimes the odds are stacked against you. Hunting pressure and factors like abundant food sources will raise hell with your coyote hunting. Unless you can legally go nocturnal yourself it can be tough.
Seems like everyone has a FoxPro these days. They are great calls, I love mine. But many people aren't good callers, they are coyote educators
Sounds like your dogs may have received their Phd's.