Do you know what? From my point of view, I think the more vocab you know, the more you understand. And when you become accustomed to the voices, you'll find out that your listening has improved. It's my own experience!

For me the background music and sound effects sometimes make it hard to understand the actors. Some actors speak very quickly and slur their words together, which makes it more difficult to understand, especially for non-native speakers.

White, I can only assume that you are still a beginner to intermediate learner. Your speaking level will be the first thing that makes it hard to understand, but there are so many factors that affect it as well. Some of it can be the sound as Susan described it, but since I know that movies are made on a very high technical level, maybe it's your own TV/computer/speakers that you are listening on that are playing a major part for you. I would definitely recommend watching with English subtitles: that will help you sooo much, both for understanding and learning. So many words and phrases repeat all the time that I find your title to be an extreme statement, most likely made out of frustration. Wikipedia says: "The Reading Teachers Book of Lists claims that the first 25 words [of the most common words in English] make up about one-third of all printed material in English, and that the first 100 make up about one-half of all written material". So if you know these 100 words, and you probably do, you should be able to understand half of the words being said, maybe not quite the meaning… As Doaa suggested, maybe you need to watch something easier, pick your movies more carefully. I recently watched "True Detective" HBO series and I had a hard time understanding everything. Maybe it's better if you watch something simpler, maybe older, maybe more traditional, like British classic crime dramas from the 70s and 80s, until you get used to the language. Or find some speeches or interviews on YouTube, TED has a ton of great ones...

Maybe it is because the actors are speaking authentic English and not classroom English. Regarding subtitles, they can both assist and hamper languag learning. When I am learning a language, sometimes I just watch a foreign movie without subtitles to get a sense of the sounds of the language. After many hours, you can learn a lot about that language without direct instruction. Subtitles also distract from the movie itelf, as it is hard to enjoy watching the movie and reading subtitles at the same time. Sometimes I will watch the same movie, once with subtitles and once without them. And yes, try to select a movie or TV show that is not too far above your level of listening comprehension. Even kids shows are okay to begin with. Sesame Street, Sponge Bob.

thank you ,you're really right i have to focus more and more on listening or what it's called "a silent period ".