If you work the bolt slowly while feeding a round from the magazine, try to see exactly where a round may be binding up. If there's a sharp edge on the receiver bottom where the round is hanging up, it can be smoothed a bit by using a stone or a dowel rapped with emery paper. (Don't hone the rear of the bolt lugs, since that could increase headspace!)
Sometimes it helps to see where the round hangs up by coating the round with magic marker prior to testing.
People are like rifles. Some are tried and true, having great eyes, personality, and fun to be with. Others never seem to hit the mark with you. Still others go off half-cocked. Still, it's nice to know most of them.