I had same problem with an AR-15, but would not lock the bolt back. Discovered that one of the bolts that attaches the gas key to BCG had broken and gas was leaking. It took a while to figure it out, but I found a video on YouTube where another guy had the same problem. I had tried other magazines and still would not operate like it should. I replaced with another BCG and problem solved. I later got the broken bolt removed and replaced and the original BCG works great. Edited: "would not operate like it should"