The only problem even close to that that I had was due to starvation. A bit of o-ring got lodged in the air passage in the AB and caused a weird sort of oscillation. The gun just wouldn't "cock" (not sure that term really fits when you're talking Vectors), the bolt just cycled back & forth slowly.
Given your circumstances, I doubt it's the same or a similar cause. Unless maybe there was some kind of debris that made it through the AB into the valve?