Your hammer may be slightly "binding" on the upper portion of the hammer slot in the grip frame or within the hammer "well" in the cylinder frame.
Disassemble the revolver. Degrease the hammer with some good solvent/degreaser that dries "clean". Paint the sides of the hammer black with a "sharpie". Reassemble and test. Disassemble and look for the "silver" portions of the hammer. They should give you a definite clue of where the hammer is contacting the rest of the gun.
Wrap "emery paper", i.e. 220 grit, around a slim flat stick (popsicle stick, eh?) and sand the tight spots of the grip frame and the cylinder frame's hammer slot. Clean very well and test again.
Sometimes one will find rather "coarse" machine work within the Hammer Well and getting it "smoothed up" is "S.O.P." to a "homeboy" gunsmith.
JMHO,
flatgate
flatgate