Most likely wear and tear on the decocking lever (sear release lever) itself, causing the timing on the hammer fall to be "late". Could also be a buildup of crud inside the gun, in which case it would require a good cleaning to restore proper function.

Wear on the slide and frame rails can also contribute to improper timing of the lever. This wear causes the slide to ride "higher" on the frame rails, which in turn causes the lever and decocker interface to be improperly timed. (hammer falls late when the manual safety/decocking lever is engaged) You may be able to test this by checking for movement in the slide to frame fit, particularly at the rear when the slide is in battery. (closed)

Replacement levers are available, but they are not considered "drop in", and must be fit to function properly.

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Ret. LE, FA Instr, S&W Armorer