Language proficiency plateau

From Teflpedia

The intermediate plateau (/ɪntə'mi:dɪət plætəʊ/) is a when a student's progress in a language significantly slows, even stops altogether, at an intermediate level.

Different learners may plateau at slightly different levels, so you might get students plateauing with a pre-intermediate plateau, or upper-intermediate plateau, or even possible elementary plateau.

Further reading: Jack C. Richards Moving Beyond the Plateau: From Intermediate to Advanced Levels in Language Learning