I agree with cayugad and would replace the nipple. Once that is done lower the hammer onto the nipple and look to see if the nipple is centered into the hammer recess. If the nipple is not pretty well centered it will sometimes drag on the side of the cap causing the hammer not to strike the top of the cap hard enough.

If this is the problem look and see which side of the hammer is off center and which way it needs to be bent. Remove the hammer chuck it up in a vice in the area where the tumbler and screw hole are located completely covering this flat area with the jaws of the vice.

Next heat the shank of the hammer red hot with a torch and with an adjustable wrench placed on the nose and spur of the hammer give it a slight twist in the direction it needs to be bent.

This may take a couple of trys to get it just right.