I am fairly new to Coyote hunting but I can tell you what my experience has been in central Ohio. I have went out at all times of the day and night. I have had as much luck from 10 am to 3pm as any time. I have seen them several times when I was out rabbit hunting in the middle of the day making all kinds of racket. I have not called one in at night yet, Or maybe more accurately "I have not seen one" when calling at night. I agree with an earlier post. "try a new sound" The last one (actually 3) I called in was with a "crow in distress" call when crow hunting at 3pm. I was standing in a fence row at the edge of a woods next to a corn field. I pay more attention to the wind direction than anything else. I think it is like any other kind of hunting.... The more you are out, the more chance you have at being in the right place at the right time. Be persistent....
Good luck!