not (one's) long suit
Not something one is particularly adept at. I'd avoid going for humor in your speech—jokes are not your long suit. Social etiquette is definitely not my long suit.
not your long suit
If something is not your long suit, you are not good at it. Note: The following expressions refer to the four suits in a pack of cards: diamonds, hearts, clubs, and spades. Tact has never been my long suit. Charm was never Michael's long suit. Note: If a large number of a player's cards belong to a particular suit, you can call that suit their long suit.