be not (one's) strong suit
To be something, such as a subject, activity, or field, at which someone is not particularly skilled or adept. Working with computers was never my strong suit, so this receptionist position is going to be a real challenge for me. Movie trivia isn't Sam's strong suit, so I think someone else should take those questions. No, patience is not Mom's strong suit—she loses her temper all the time.
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