Was the woofs downwind of your location?If you were 24 minutes in I would say that was enough time for them to circle probably out of your sight and pickup your scent.Don't over do it with the calling.I can't say you did anything wrong,they are just extremely good at figuring out whats there and not being seen.Try not to give them a good downwind route,use roads,buildings even big open fields that you can see across to help with that.If there is a slight hill ,fencerow etc. downwind they will take advantage of it to circle and wind you.I think guys call in more coyotes than they realize they just never see them.Welcome to the thrills of coyote hunting.Probably the toughest critter out there to get a shot at.