Yes, I’ve hit those plateaus with shamisen many times, where it feels like you just aren’t improving any more. It’s tricky, especially if you don’t have many chances to see players who are better than you in person. Eventually you run out of new things to try. These suggestions are all about doing things other than just practicing more.
One thing you can do is more fully develop your ear: increase the quantity and quality of your listening. Have the music playing in the background when your doing other things, and also spend some time doing active, deep listening as well. I’m guessing you play Tsugaru? Try listening to recordings of great players with an ear for what they are doing that you are not. You could take this as far as transcribing recordings, slowed way down, and seeing if new things come out of the pieces this way. Do you listen to the old players like Takahashi Chikuzan or Shirakawa Gumpachiro? If not, maybe try to develop a taste for that playing, and listen for the things that they do that younger players don’t.
You probably already play some min’you, but you could branch out further and try to play other genres like nagauta or kouta that have a completely different feel. The rhythms and scales are different, and the shamisen and bachi are held differently (with kouta you use your index finger), and the stroke is much lighter. There are a lot of subtle differences that can be interesting to explore and add to your tool kit.
You might even take up a new instrument like yokobue or shakuhachi and learn some of the pieces you already play on shamisen. If you don’t do this already, you can learn to sing the songs that you know on shamisen. Shibutani Kazuo told me many years ago that to really play Tsugaru shamisen you need to sing as well. That may or may not be true, but at the very least, singing those songs gives you a new perspective on them. All of this will teach you things about those pieces that will make your shamisen playing richer. Learning to sing or to play other instruments is a long term plan, and will take you away from the shamisen for stretches, but sometimes that time away lets you come back to it with a new energy.