Out of your comfort zone
Just like with working out your body will reach a plateau…when that happens you need to mix things up, change your routine, analyze if you need to increase weight or reps.
Your body gets comfortable and you’re just going through the motions, getting a momentary pump which defaults after a couple hours and no lasting progress is made.
A muscle needs to tear in order for new muscle to grow in it’s place, hence the term “ripped”.
I know a lot of people who when learning a language this has happened to them, and noticed this with many of my language students a while back.
You’re going through the same routine, learning the same sequences, grammar, phrases, conjugations, but aren’t really stretching and getting out of our comfort zone.
Like with anything, unless you’re progressing you’re either stagnant or going backwards cause others are moving forward.
Unless you’re doing doing things that may be uncomfortable and embarrassing, that force you to step out of what you’re used to, like a muscle; your growth in a language will stay put.
Initiate conversations, get a language course which focuses on speaking. Get out of your comfort zone, possibly unpleasant at first... guaranteed lot’s of fun in the end.