I purchased this gun used about a year ago. I've taken it to the range a few times with no problems. When I took it out yesterday, it would fire one shot but wouldn't cycle the next round. The bolt carrier group would not come back after the weapon was fired. This leads me to believe there is a problem with the gas tube or gas system. Is it possible for a gas tube to wear out or develop a crack somewhere? How should I go about trying to fix this problem.