Could be lots of reasons it would not come in. It could have seen you enter the area, seen you move, heard you, smelled you or maybe heard the calls you are using before. It may not have been hungry. It also depends on the age of the coyote. If it was a young coyote, the howls may have intimidated it and it was afraid to approach. This time of year there are a lot of juvenile coyotes out on their own and they will shy away from more mature coyotes (and their howls). Unless you are very experienced with howling, it is best to stick with the distress calls, especially this time year. The howling may work better starting in January when they are starting to breed. Try some different distress calls. I called a couple coyotes in the other night with kitten in distress.
Don't feel bad. This happens to me all the time. It is frustrating, but just part of the challenge.
Tim