Well, I'll try to answer the last part of your question first! If you are positive that the area has not been called for a year or more, than they should not be call shy, and should respond to a call. By changing calls, a hunter should be able to call the same area two weekends in a row and have success. By changing calls, I mean, for example, change from only using distress to only using howling. You can't expect to call the same coyote by only changing distress sounds, usually. But, by changing sounds, changing your set up location and time of day will all help fool a coyote more than once.
Why coyotes howl and not come to the call some times is a question that many veteran coyote callers would like to know. We are learning "what the coyote is saying" when he howls more and more. So, until we know what they are saying when they howl, we can only guess that, 1) they have heard that tune before and aren't falling for it again, 2) they busted ya when you came into the area, 3) they simply aren't interested in what you had to offer! etc. etc.
Don't give up on the area that you obviously knows holds coyotes, such as where you heard them howl, instead, next time try to change your calling location, how you approach the area, and change calling sounds.
If the coyotes haven't been called in and shot at, they can still be called in. It is just a matter of offering them what they want. This time of year, howling may be the best calling sound to use, BUT, when you use howls to call a coyote, you need to say the right thing, because you are now talking coyote language, and if they don't like what you have to say, then, again, they may respond, but not come. Also, pup distress should work well this time of year too.
It can be frustrating sometimes, when trying to fool 'ol wiley to come to you, but IMO, there is nothing that compares to it, when it works. Often times, the first coyote is the hardest to get, once you start having success, it gets a little easier, and when you fail, you can blame the coyote and not yourself and your techniques.