It is common for advanced language learners to reach a "plateau" where it is hard to make progress. So you are not alone! For a formal lesson, I would recommend using advanced materials that challenge you with new vocabulary, phrases, and grammar. For a discussion-based lesson, I would recommend reading an advanced article together and then discussing it.

Hi Nao It does happen to many students who had reached advanced level of language. You then have to push yourself a rank up. Make sure you use extreme adjectives instead of adjectives or start using formal instead of informal or semi formal. Every conversation with a good tutor should result in some interesting new words or vocabulary.

You can also find a language partner, so that you can speak with them and learn new words

You might also benefit from having someone review your written work. For example, I would write your message as follows, but there are of course many ways to write this. I don't feel like I am making progress in English as my italki lessons seem to merely reinforce my existing English vocabulary. Please share with me any ideas you have for using italki more effectively.

Hi Nao! I'm a professional English teacher with a structured curriculum for every student. I offer both task-based lessons (using worksheets containing various exercises based on interesting, real life topics) and conversation lessons (also structured according to different topics). All the lessons provide for vocabulary-building. If you'd like to learn with me, visit my profile :)