Definition
This idiom is used to express that when one unfortunate event occurs, it is often quickly followed by a series of other unfortunate events, making a difficult situation even worse.
First, I missed my train, and then I discovered my wallet was stolen; it never rains but it pours.
She lost her job, and then she was diagnosed with a serious illness; it never rains but it pours.
Just when we thought things couldn't get any worse, the company announced layoffs. It never rains but it pours.