i'm sure this has been covered a million times but i just thougth i would post this.
i was shooting today with one lower and 2 uppers, swaping back and forth. each upper was having issues with light primer strikes and flat out not firing even with a pretty hard strike, with ammo that was known to be good. xm193 and IMI
so after changing bolts and fire pins around it occured to me the hammer was not striking hard enough. luckily i carry extra ar15 parts and tools in my range bag. i swapped out the hammer spring and reloaded the cases that would not fire. all rounds went bang and all ammo there after was good as well.