Because the teacher is probably speaking slowly, and clearly, and deliberately avoiding jargon, slang, and ambiguity. Those are unfortunately parts of normal speech... You have probably reached another step in your learning, when you need to start getting more familiar with those extra complications.

it's normal, it's called exposure; when you're learning the first voices you hear in the target language is the teacher's. But he/she is trained to talk to you in the simplest way and clearest tone possible. Then you watch people talking in other contexts and your brain is confused because it doesn't recognize all the patterns. But it's a great exercice. Keep watching videos and listening to people other than your teacher even if you only understand a few words. Want to take it to the next level? go watch on youtube different english accents... What does and Australian or Scottish sound like? your brain will be confused but in the long run you'll be prepared for the real world :) good luck!

Because that TV show was totally boring.So your brain stopped to understand. Haha I recommend watching your favorite movies in English no subtitles .May be you can catch and understand more words.learning with situations are good to memorize.