If you use a command too many times improperly; it won't work. An example would be the word come. Calling a dog to come and then he doesn't, just means that to the dog it has no meaning. The way to fix that problem is to start training using a different command. I use the word "here" for come or if my lab is running around and I want him to come in I just ask him if he wants to eat. When he comes in the house, he knows I will head straight to the refrigerator to get him a treat (which to a lab is the greatest thing, next to running free). Reinforce the new command in an environment where you have control of the dog such as on a leash. If he doesn't come, gently pull him to you. If he does come, reward him.