When I apply the slide mounted safety/decocker on my S&W Model 3904 the safety works as the trigger cannot be pulled, but the hammer stay back in the single action position (cocked). The safey works in DA/SA but it will not decock the hammer? Any ideas what is broken?

Probably the sear release lever in the receiver is broken, worn, or the engagement surface was filed too much by someone.

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VCDL, GOA, NRA

Thank you.The DA/SA trigger still works and the trigger pulls seem normal.

So your decock doesn't work anymore...Now what?