most likely it is some sort of feed error from 1 or more mechanical errors occuring. I would guess it is a weak spring in your boltcatch on the side of your reciever. Personaly I would replace the bolt catch spring and it would be smart to replace the boltcatch button itself. Next check your bolt, examine the fins and look to see if there is any excessive wear around the portion that hooks onto the boltcatch tab. I suspect it is either of these 2 things. The bullet not firing on the 2nd shot is a classic case of a bad firing pin and/or bad ammunition. If it isn't extracting the bullet then examine the extractor and it'sspring.
What I would do is borrow a friends bolt, shoot a few rounds to see if it is in the bolt. If same thing occurs. Use your bolt in your friends gun and check. Generaly with 2 guns you can find any problem...