My man Sheridan often uses the saying, something along the lines of "the red-tailed hawk only catches a mouse on one out of ten attempts." (Sorry Sheridan for stealing your line, hopefully I didn't botch the wording too badly).

Coyote calling is a balancing act of quality and quantity. You need to make as many GOOD sets as you can. You scout as much as you can, figure out where to dogs are bedding/denning, where they're feeding, where they're drinking, and how they travel. Design your sets to captilize on those behaviors. Get in and get out. If you call for a half hour and nothing shows, move on. I can't think of the last time I spent an hour on a set unless it was just the last one of the day and I didn't have anywhere to be and didn't have any dogs on the truck to get skinned (read: unless I was desperate).

Rule number one for coyote calling: Don't get discouraged.