The most common reason for bulls getting away once committed to coming to our calls is they reach a certain point & hang-up! They hang-up for various reasons, first & foremost, they stop & scan the area visually where the sound/sounds came from. They can pinpoint where they came from by a matter of feet. It's true when there's a shooter/caller tag-team a shooter can be in front of the caller, distance may vary according to terrain, cover & wind & calling sequence or encounter. Anytime one setsup for a calling senario he should choose spots where elk cannot stop & look from out of bowrange shots. That is very important, elk must come close enough to investigate the sounds, they only do this when they cannot see where they're coming from because of obstruction.

You don't say if you were using bugles cow calls or both in trying to coax elk your way. This will make a difference in how to handle particular encounters. At times when there's 2 hunters the caller needs to start pulling back & going away calling sparingly as he does as if he were leaving. Snapping branches & light stomping as he retreats, this can pull a bull right passed the shooter.
I would say from your comments you were using more cow calls than anything once you were working those bulls to the 80-100yd range. If so, what happens is a bull will stand back & try to call this cow to himself. He may start with a high pitch bugle which is asking thios cow to come on over, if she doesn't show he in most cases will come closer but he'll get more demanding in his sounds such as the high short pitch scream with a light growl added & or chuckles at the end of his bugle, this shows he is getting demanding for this cow to come now!! Unless you show him why you are not coming he'll get nervous & bugger out of there as that most likely wasn't his first rodeo! (grin)
When I'm hunting & this senario takes place I will normally throw in some bull pants & huffs, this shows an intruder bull is coming on the scene & most likely the cause for her not coming. You can really play it up once you inject this into the equation! Creating a skirmish creates excitement ! There's tons more to be said, but you get the idea! Bulls hang-up for a reason, as elk hunters we need to know how to combat them before they ever happen so we are ready for nearly any encounter! Understanding what elk are saying is a huge key to ones consistent success, once you can do this you can tailor your style & next move to "fit" the encounter.
Next time also consider Call & Stalk approach! Leave a caller behind to call every few minutes to keep bull vocal as a shooter coyotes his way to the unsuspecting bull. Yep, there's tons of ways to handle an encounter! (grin)

ElkNut1