The condition is caused by damage or wear to either the SA bevel on the trigger, or the SA cocking notch on the hammer, or both. Modification of the mainspring or the rebound spring (or both) can also contribute, as well as dirt and debris.

If the hammer notch is in-tact and has not been tampered with, you may be able to restore function by sharpening the trigger bevel back to the factory prescribed angle. This is a job for someone that has the proper stone, and knows the correct procedure. If the hammer is damaged, it will have to be replaced. You can't restore a damaged hammer notch.

If the gun came to you used, I would also suspect that the OEM rebound slide spring has been either cut or replaced with a lower poundage, aftermarket one. Replacing the altered rebound slide spring with a new, OEM spring may restore function if the trigger and hammer surfaces are undamaged.

If you cannot find a trusted, trained gunsmith in your area, I would agreee that sending it back to S&W is probably the best way to have the issue resolved.

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Ret. LE, FA Instr, S&W Armorer