Don't think your dog is understanding your command. Maybe they are just stubborn and will never listen to you. Let's set up the scenario - Your want to train your dog to sit. When you give the command you say sit, sit, siiiiiiiittttt. Finally your dog sits. By now you are probably a little frustrated. Next time you tell them to sit you do the same thing again you say sit, sit, siiiiitttt.
What have you successfully taught your dog? That they should wait until you say sit, sit, siiiitttt to actually perform the command. Your frustration is letting the dog know that sit isn't such a great thing to do. There is no reward and they will not want to repeat the behavior.
What do you want to teach your dog? That when you say sit once that is the command.
How can you achieve that? You will need to use your command only once. If you say the command more than once, then they dog will learn that you need the word said three times before you sit. Give the dog a moment to think about the command especially when you are starting out. If they don't respond try luring them with a piece of food into the position you want them to be in. If they still don't respond just forget about the command and don't reward with food. You may have to take a step back in the training to ensure the dog understands what you want.
Keep things positive so that when your dog sits he/she will know that that is what you wanted them to do. They will want to repeat the behavior again if they are rewarded.