When something goes wrong and the rifle won’t fire, the first question should always be, “What changed?” Before answering that, we have to determine—or at least I determine—whether we’re talking about a “fresh” rifle going through its shakedown period, or a (previously) trusted gun that’s suddenly decided to stop running. If it’s the first scenario,

Source: Six Troubleshooting Solutions for AR-15 Function – Patriot Gun News