Possibility that the rear gauge, or headspace, is out of specification. Did you check for excessive end shake on the cylinder? This can cause the cylinder to move forward enough to limit contact of the hammer nose and the shell casing/primer.

End shake on the yoke can also contribute to the rear gauge opening up to the point where misfires occur. This happens when the interface between the yoke screw and the button on the yoke stem becomes worn or corrupted, causing the yoke itself to move forward and back slightly when the cylinder is closed. If the yoke screw pilot and the button on the yoke are fit properly, the yoke should open and close with very slight resistance, and with no"gauge" or movement in the yoke itself when the cylinder (yoke) is in the closed position.

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Ret. LE, FA Instr, S&W Armorer