The plateau is the most common place for English learners to stop, and the most discouraging, because nothing is going wrong. You can read, follow a film, handle most situations and make yourself understood — and you've been at roughly that level for two years.
Why it happens
The methods that took you from beginner to intermediate stop working, and nobody mentions it.
Early progress is fast because almost everything you learn is immediately useful. Learn a hundred common words and your comprehension jumps. Learn the present perfect and whole conversations open up. The gains are large and visible.
At intermediate, that stops. You already know the frequent words and the main structures. What's left is a very long tail: less common vocabulary, idiom, register, the difference between two words that both technically mean the same thing. Each item gains you much less, so the same amount of study produces far less visible change.
The second reason, which is bigger
Intermediate is the level at which you can get by. And getting by removes the pressure that drove the earlier progress.
You have a working set of maybe five hundred words and a dozen structures that cover most of your life. You use them, they work, and you stop reaching for anything else. The language stabilises around what's sufficient.
This is why people live in an English-speaking country for a decade without improving much. Exposure isn't the variable. Demand is.
What actually moves you off it
Production, not input. At this level, more reading and listening largely consolidates what you have. Speaking and writing are what force you to find what you're missing — because the gap only appears when you need a word and it isn't there.
Being corrected. Fossilised errors are the signature of a plateau: mistakes you've made so consistently for so long that they feel correct. You cannot find these yourself. Someone has to tell you, repeatedly, because by now they're habits rather than gaps in knowledge.
Harder material, deliberately. Not what you can follow comfortably — what you can follow with effort. Comfortable material maintains; difficult material extends.
Narrowing the target. "Improve my English" is unachievable at this level because it isn't a single thing anymore. "Handle a client call without rehearsing" is achievable, and progress is visible.
What to stop doing
Beginner activities. Vocabulary apps, grammar exercises, and relistening to things you already understand all feel productive and mostly maintain your current level.
They were the right tools eighteen months ago. That's the whole difficulty with the plateau — it isn't caused by doing the wrong thing, it's caused by continuing to do the thing that used to work.
How long it takes
Longer than the first stage, and that's normal rather than a sign of failure. Going from beginner to intermediate is a few hundred hours. Intermediate to genuinely fluent is measured in thousands, and most of it is in use rather than study.