drop behind
phrasal verb with drop verb uk/drɒp/ us/drɑːp/ -pp-
落后，掉队，跟不上
to get further behind or away from something or someone
As the pace quickened, Pepe began to drop behind. 随着速度的加快，佩佩开始跟不上了。
She stopped going to classes and dropped behind in her schoolwork. 她不去上课，功课也就落了下来。