Many ESL/EFL students experience a slowdown in their language learning once they reach an intermediate level of language proficiency, and they often do not know why. Although there are a number of factors that contribute to this, a main cause can be a 
fossilization (a state of not changing) of their language skills. In other words, at the beginning stages of language learning, students are focusing on everyday topics like shopping, eating out, and directions; however, as levels increase, students need to be able to talk about more abstract topics or subjects (e.g., freedom, honesty, psychology)  that require critical-thinking skills (e.g., compare, analyze information from various sources, infer meaning); unfortunately, students tend to try to rely on their basic language skills to talk about advanced topics instead of experimenting with newer language skills they are learning at the intermediate level.
What complicates this matter is that most people, even in their native languages, tend to use basic language on a daily basis---you can't change that---and might not find themselves (or try to put themselves) in more challenging situations, particularly when learning a foreign language.
So, what other factors affect the slowdown of language learning among students? Share your own personal experiences and solutions to this issue. Randall