Need a little more info. Is this factory ammo? Is it russian ammo? Reloads? Primer might not be seated good. I was having this problem with 7.62x39 in an ar using russian ammo and seated the primers a little more with a hand primer and cured the problem.
reread your post and it appears you reload. How do you seat your primers? What brands have you tried? Have you segregated the brass that didn't fire to see if it is the same brass each time? Primer pocket could be too deep. Have you pulled the bullets on loads that didn't fire to ensure they did have a powder charge. Do you tumble your brass? Primer pocket plugged. Wet tumble? Might have water still in case. Have you checked your firing pin protrusion? May be set to short.
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