Weird Airsoft Loading problem?

I have a VB M14 Sniper rifle...I love it, and I bought it a few weeks ago. There's one problem, though. Sometimes, when I fire, I get a long BRAANG! sound from the spring, and no BBs are shot out. The next time I fire, I don't get that sound, but two BBs shoot out. Can anyone tell me what this means/how to remedy it?