How to keep my dog focused on training?
When you are teaching commands or basic obedience to your dog, always start training in a low distraction area. You cannot expect your dog to perform when there are a lot of things going on around him. There is no way that he can focus on you luring him to sit while another dog is trying to play with him. He cannot come to you with a child running around screaming. The dog is not being disobedient; he is simply more motivated by what is going on around him rather than focusing on you.
You need to start in small steps.
When first introducing the command, begin in a spot where there are zero distractions. Once your dog is proficient there, then you can start teaching in other areas. You cannot expect your dog to learn a command while at the dog park playing! Start in your home, then your yard, then the street and so on.