From the looks of the nipple, it appears that the hammer cup is not at ninety degrees to the axis of the nipple. In other words, the hammer may not be striking the nipple squarely. That, over time, will bend the nipple and put more strain on the contact side of the nipple while the other side gets no strike. Examine the cup of your hammer to see if there is a complete circular indent from the nipple. If not, adjust the inside of the cup with an end mill (I made mine from a drill bit, and can post pics again, if that'd help)