I'm reaching back into memory a bit, but I'd have two suggestions:
1) Take the bolt out and flick the safety on & off a number of times. You'll often be able to catch the cylinder that blocks the firing pin hanging up in it's housing. Because it passively falls away when the safety is disengaged, any little burr on either the cylinder or hole with fooch it, causing the firing pin to only travel half-way, and resulting in a mis-fire.
2) IIRC, there are two other such cylinders (all connected) that can be messed up the same way -- one that locks the bolt( under the handle) and one that blocks the trigger (in the front). They both need to be checked too, but #1 above is the major culprit, IME.
