Are you loading to spec.? Sounds like you may need a heavier spring, but that may cause you to not have enough energy to drive the bolt back far enough to grab the next round. If they are feeding fine single, I’m guessing it’s a gas issue. A lighter spring may also allow the buffer to travel rearward faster and, may help fix the feeding issue. Does the bolt lock open,when feeding one at a time? Can you get or use an adjustable gas block? Sounds like you are going to get some range time to try to figure this out..