Trapper. Lets verify that you have clearance. When you pull the trigger the sear slips off the ledge of the trigger. As the force of the trigger spring pushes the top of the sear down the bottom portion that contacts the trigger makes an ark that follows the large radius of the trigger.

This must have clearance. Verifiable Clearance as in feeler gauge or coke can thickness throughout the travel. It is obtained by backing out the screw minute amounts if there is none. This is important because it can obstruct the movement of the firing pin. If it is not that then you have an obstruction in the bolt or someone has lightened the spring by cutting it or adjusting the cocking piece end or the firing pin spring. I hate to keep pushing this but I want to make sure this is not the problem as you have ruled out head space. If you do verify the clearance and still have the problem, then you need to look at your bolt.