Just took my new AR to the range today, first round fine, then "click". It ejected the brass fine but did not strip another round out of the magazine. Every time I had to manually cycle it to strip another round. Someone there gave me some oil, I oiled the bolt and it worked ok for a little while then it would go back to not stripping a round after firing. A few times the bolt got stuck halfway after ejecting the brass. Any ideas?