Some people speak so quickly that you are just not hearing all of their words. They also are not speaking clearly. If you knew the language well, you might catch more of the words, or understand what they are saying even if you did not hear all of it. You can easily ask people to repeat what they said (if it's not in a movie you're watching). When they repeat their sentence, they may speak up more. Ask like this:
Please repeat what you said. Will you say that again? What did you say? I didn't understand some of your words. Will you say that again, only louder this time? etc.