Most English learners in Vietnam hit the same wall: they understand what they read, but not what they hear.
A word has three parts
When you learn a word, there are at minimum three things to learn:
- Meaning. What does it mean?
- Use. How is it used?
- Pronunciation. What does it sound like?
In most cases people learn the meaning and the usage. Nobody learns the sound. So you have no idea how the word is actually pronounced.
Two reasons listening fails
One: you genuinely do not know the word.
Two: you do know it, you would recognise it instantly on the page, but you do not know how it sounds. When it goes past your ear, you do not realise that this is the word you already know.
The second case is the common one, and the most frustrating. Your vocabulary is not lacking. It is simply stored in a form your ears cannot retrieve.
What to do about it
Go back and learn the pronunciation of words you already know. Once you know how a word sounds, you can recognise it when it arrives, and only then can you understand it.
This is why someone can study English from year one through year twelve, plus four years of university, and still dread listening. And if you cannot hear it, you will never open your mouth.