I have found that primmers can be a problem but in most cases a light strike is caused by either dirty firing pins, worn hammer spring, or more likely, the cases weren't re sized and trimmed to spec. The bolt won't close completely, as a safety factor the bolt has to be completely closed or the firing pin is unable to strike with full force, when the hammer hits, it will close the bolt completely and so when its re cocked and fired it goes boom. The next time it happens wait for a few seconds and then extract the round and check the primmer for depth of pin impact if it is not as deep as one from a fired round it may be length and sizing.