If a certain thing doesn't go wrong, another most probably will. * /When John lost his keys and his wallet, and his car wouldn't start, he exclaimed in despair, "If it's not one thing it's another."/ Compare: ONE DAMN THING AFTER ANOTHER (ODTAA).
IF IT'S NOT ONE THING IT'S ANOTHER
Meaning of IF IT'S NOT ONE THING IT'S ANOTHER in English
Dictionary of English idioms . Словарь английских идиом. 2012