WE HK416 Firing Problem - fixed

[-Trooper-]

So my WE HK416 (ver 3) has decided to act up. It was perfectly fine yesterday but today it just won't fire properly. So here is the problem. I load the bbs and fill the mag with gas and put in it the gun and pull the trigger. Sometimes it fires the first shot and then stops or it doesn't shoot all together. When I pull out the mag, I can see that the valve knocker hasn't moved to the full forward position. So what is exactly causing the valve knocker from moving fully forward?

Thanks

Added: I did some testing tonight and when I charge the gun and pull the trigger, the valve knocker doesn't seem to move forward anymore.

Fixed. I did some tinkering found out that the rubber stopper was out of place.