seems like everyone is struggling with getting the shot off after calling a bull in, to me that is a setup issue, get close, get a good setup, and call, if you can't get the first two then hold off on making noise, mature bull or raghorn the program doesn't change raghorns give you lots of slack, you can have a mediocre setup and still do ok, big bulls tend to find the hole in your setup you didn't know you had, you're probably only getting one good chance, best to choose when to use it carefully...
I find it just as easy, if not easier, to kill bulls calling solo, having a caller is great in certain situations but most of the time if you are by yourself it's far easier to adjust as needed to get the shot, same cardinal rules apply, don't call if you are not ready to shoot, don't call from a spot that you are not going to be able to shoot from, don't call if the bull can easily circle downwind without you seeing him...
As far as early season calling in mature bulls, not just any bull, better to be lucky than good, the three biggest bulls I've called in have all been completely random, one came in from about 500 yards to a single bugle in about 30 seconds, he bugled, I walked to cover and knocked an arrow and bugled and he was in my lap, shot him at 50 yards in very open terrain, the second I had an hour-long back and forth finally getting into really thick timber with the bull, I shot him at 5 yards within maybe 2 seconds of seeing him for the first time, the third came in over the course of 4 hours from probably close to a mile, I was tagged out and hunting with a friend, we could hear a bull way onto the adjoining private land, given that it was noon we figured we would just occasionally call and see if we could get this bull to wander our way, after a couple of hours it was clear he was coming our way and getting more excited by the minute, we kept amping it up as long as he was responding, he finally broke loose and charged in, unfortunatly just in time for a couple other hunters who had heard our "rut" and were sneaking in on us to bump into him...