Check the main spring strain screw. It may be loose or someone may have shortened it for a lighter trigger pull. I have run into this issue on a few used guns I have picked up. Remove the screw and check the end for metal removal. To fix one old M15 I had, I removed the anvil from a spent large rifle primer and used it for a cap on the end of the screw. Fixed my misfires. Later I just replaced the strain screw