There is only an intermediate plateau, when you study the same material over and over again. As @chytran said, study harder material. The ‘sweet spot’ of New Words differs, depending on your level. But you want something that’s not too easy and not too hard. Eg. 8-15% New Words is good at my current level, but as I beginner, a lot of the lessons were 30%+. So it changes depending on your level (and if I start with reading-while-listening and skip the pre-reading-only). At an intermediate level, you should be using mainly content for natives, or at least half the content should be content for natives. YouTube videos, podcasts, and some TV shows are good. Some content is easier than others, so you’ll just have to try out each individual content for yourself. If you do this, then eventually move onto books, you just won’t experience the so-called intermediate plateau.
You also need patience, because it takes a long time. It may just feel like you are not improving, but really you are.