I'd call more aggressively, and might stage a fight with fighting purrs and putts on a couple of pushbuttons and a diaphragm. What have you to lose?? If they're walking away, maybe it is because you have lost their interest. As they say, take their temperature. If they are hot, keep 'em hot. The time for one call and you are done is when the toms are lukewarm, but interested. If they are gobbling on top of your yelp, yelp again. When they are clearly heading your way, then slow it down SOME, but keep 'em interested.

(I've seen 2 year olds, though, that gobble like mad but won't actually come in. I think that it is because they're uncertain as to what to do, or they are not the boss, and are afraid the boss might show up. )

Also, it often pays to stick it out until at least 11:00 a.m. I've killed a few toms that gobbled at my yelping in the early morning, gone away (usually with hens) and then come sneaking back in at 10:00-11:00, when they were done with the hens, but still feeling their oats.