One of the best training methods I've been shown is to play hide and seek with your boxer. If they hear you, but don't see you, their curiosity overtakes them and they'll come running. When they do, plenty of doting and praise and a treat or two will make them want to come whenever you call them, unless......
he thinks responding to your call will mean a loss of freedom. You call him/her when you are going out and the dog will be confined. You call him to put him in his crate. It's a Pavlovian response. If being confined, or cooped up is what happens when he comes it will naturally make him reluctant to come. A good trick is to call and then give the dog its freedom. Call him to pet him or play with him. Make it worth the dog's while to come to you. Do this often and he will respond more positively to your command. Make it more interesting for the dog to be with you.
My wife and I play the hide and seek game with Indy. One of us will hold him while the other goes out of his sight. Then we call Indy and he comes running. Then the one who held him will go to another area and call out to him. He will initially go to the last place he saw us, then begin to search for the caller. We call him again and he usually pinpoints the source of the call and runs with great relish to find the caller.
He really seems to enjoy this game. When he finds the "hider" he gets showered with affection and praise. So if your dog is called more frequently for things he enjoys rather than confinement, or something he considers unpleasant, the hard part is over.
Regards. Tom Liggin