it never rains but it pours当有麻烦的时候，麻烦的事儿往往都扎堆来。引申为“祸不单行”，有时也可以理解为“不鸣则已，一鸣惊人”。

【例句】
I broke a fingernail, laddered my tights and then missed my train– it seems it never rains but it pours.
我弄破了我的指甲，紧身衣裤抽了丝，还误了火车——真是祸不单行。

First Tom lost his job, then his wife fell ill; it never rains but it pours.
先是汤姆失业，后来他妻子又病了，真是祸不单行。