Could be any one of/or many reasons. 1)maybe there weren't any close enough to hear the caller, 2)maybe they were there but weren't interested enough to come, 3)maybe they did come but winded you and left before you saw them, 4)maybe they have heard that song before so decided not to come to the dance again, 5) maybe you were looking the wrting direction when they were there and they left without you seeing them, etc...etc...,
It may take you several sets of calling before you actually call one in, don't expect instant success, unless the coyotes are really thick in the area and are having a hard time finding food!
One critter per 20 sets is not uncommon for many callers.