There could be lots of things responsible for the misfires. You could have headspace issues with certain ammo, a weak hammer spring, broke or worn firing pin, or even a transfer bar issue. Make sure you hold the trigger fully rearward when you fire every shot. If you are letting up on the trigger the transfer bar could be dropping when the hammer strikes. If you are sure you are holding fully rearward, the next thing I'd check is firing pin protrusion. Open the action then cock the hammer and pull the trigger fully rearward with your finger while lowering the hammer with your thumb, measure the protrusion of the pin with the hammer fully down. Sometimes the pin will wear, break, or even bend causing the occasional misfire. How deep are the dents in primers that did fire?