The first months are full of obvious wins. Then one day you notice you have not felt better at this in weeks. Nothing has gone wrong. You have simply crossed from a stage where progress is visible into one where it is not.
The plateau is mostly a measurement problem
Early on, every session adds something you could not do yesterday. Later, the gains are distributed: slightly faster retrieval, slightly fewer hesitations, slightly more natural word order. Real, but invisible day to day. Record yourself speaking for two minutes every month. The plateau usually disappears the moment you compare February with November.
You have stopped doing hard things
By the intermediate stage you can get your meaning across, so you stop reaching. You reuse the same fifty structures, talk about the same safe topics and never find the edge of what you can do. Deliberately choose subjects you cannot yet handle: an opinion you would struggle to defend, a story with a complicated sequence of events, an explanation of your work to someone who knows nothing about it.
Beginner material has stopped teaching you
Courses are dense with new material at the start and repetitive later. If your lessons feel easy, they are no longer the thing moving you forward. Swap structured material for real material: a podcast made for natives, a novel, a conversation with someone who will not slow down for you.
Narrow your target
“Get better at Spanish” cannot be finished, so it never feels achieved. “Be able to describe my job for two minutes without stopping” can be, and it will be, within a fortnight. Small closed goals restore the sensation of progress that the early stage supplied for free.
The plateau is the price of leaving the beginner stage, not a sign you have stalled. Change what you measure and what you attempt, and it ends.