Some times the dogs just aren't there, or aren't hungry enough to come in to the sound of an easy meal. Years from now you'll probably have dry spells. It's easier to understand and explain after you've actually had some success calling though, so I can appreciate where you are at.
When you can, try to scout areas before you actually hunt them. Look for well worn trails and sign, try to figure out the travel habits of the animals. Like Jerry said, look for a stand location where you can have an elevation advantage and see the likely approach avenues. Once you've placed your e-caller, or sat down to use mouth calls, just set there for a bit and survey the area really well to try to figure out where an animal could surprise you, and to become familiar with rocks and bushes. Be sure to set in front of something like a bush, not behind it. It will help break up your outline, and you'll be able to see better. Set really still, scan the area by very slowly moving your head, no sudden movements. If you end up with crows or magpies flying really low over you in response to your call, and you don't spook them, it's a good sign. It also makes a scene look more natural to a coyote. Be on the lookout for magpies coming in. Some times they are following a coyote, looking to maybe clean up whater the coyote is going after. If they are in a group circling you, from my experience there usually isn't a coyote really close by. If one or two are coming towards you, but landing from time to time, or circling some distance away from you, they could be following a coyote. Crows are especially sensitive to motion. If they hang out close to you, chances are you're staying still enough.
As others have said, scent is pretty much impossible to hide from a coyote if they get downwind of you. You can take easy steps to help overcome that though. I keep my hunting gear (I usually use a sapper type ghillie) in a bag with some sage brush. If I'm using a decoy I will sometimes use a scent (like rabbit on a stick), on the decoy itself. It's my belief that the decoy helps overcome some fear if you do get scented, and helps coyotes drop their guard and get excited in general. Add a little scent to it and I think it helps a bit more. Realize that most often, if there is wind, a coyote is going to try to get downwind of you and/or your call. Try to make sure you can see the routes they may likely take to get downwind so you can shoot them before they get there.
Depending on how you've set up, there is a chance that you've had coyotes come in, but they have held back and busted you without you seeing them, and slowly went away. It's hard to understand how sentive they are to motion. They are predators that often eat very small rodents, and their vision and hearing are highly developed. I like a bit of wind because if you set in front of some vegetation that has some motion because of the wind it makes any movement on your part blend in a bit more.
Don't give up. When you've had a successful stand make sure you really take in what the area is like, and what was right about the stand and why it worked. The learning curve can be really quick.