It's probably "out of time", meaning the hammers are not starting to cock soon enough, as the gun is opened. The fix is to restore the loss of proper drive to the cocking mechanism parts. As to whether you can fix it yourself, I can't answer that for you. Bending or peening the parts is a touch and try operation. Go too far and the cocking parts will prevent the hammers from gooding a good lick on the primers. If it's an old gun with lots of use, the above is more likely than the primer brand, IMO.