It never rains, but it pours.

祸不单行！

讲解：

祸不单行还有另外的表达：misfortunes never come singly；One mischief comes on the neck of another.

eg: First my car broke down, then I lost my key: it never rains but it pours!

先是我的汽车出了故障, 接着又丢了钥匙,真是祸不单行!