It never rains but it pours / Adding insult to injury
Literally 'to rain on what is already wet'. Used when bad things keep happening to someone who is already having a difficult time.
First the accident, then the redundancy. It never rains but it pours.
—They've also raised my rent. —Wow, it never rains but it pours.
When things go wrong, it never rains but it pours.