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
Slang English vocab. Английский сленговый словарь. 2012