The cocking lever is either worn or bent, or the hammer toes that engage the cocking lever are worn or bent. Flues guns are notorious for this. The hammers are cocking too late due to the wear and the firing pins are dragging in the primers as the barrels try to open. This can be adjusted back to proper functioning via TIG weld and refitting by a competent smith. Be forewarned not to try to "bend" these parts back into compliance, they are hard and will most likely break, compounding the issue.

Best, Dan