Usually it's the SA bevel in the trigger that is worn or has been modified in some way. Could also be caused by damage, crud or wear to the cocking notch on the hammer. If the SA measures below approx 3 pounds, that's an indicator of a problem.

First, check the rebound slide spring to see if it has been cut or is a "lighter" weight aftermarket spring. Many times, just installing a full length OEM rebound spring can solve the problem, and bring the pull weight back up to 3 pounds. An altered mainspring or mainspring strain screw can also contribute to this.

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Ret. LE, FA Instr, S&W Armorer