In English, the idiom "when it rains, it pours" is used to say that when one bad thing happens, other bad things often accompany it.
From the Merriam-Webster dictionary's online example:
The team not only lost the game but three of its best players were injured. When it rains, it pours.
Is there an equivalent idiom in Italian?
I found one source suggesting this might be equivalent:
Piove sul bagnato. (tr: "It rains on wet ground.")