Just came back from a range session.  Had a few time where the 9mm carbine I build wouldn't fire.  

It happened 2x and 2x on the last session.  It would happen randomly it just suddenly you squeeze the trigger and nada.  The bolt moved back and chambers a round but when you squeeze the trigger, nothing.  When you flip the safety switch from fire to safe and back to fire, the 9mm carbine will shoot  continuously again.  The carbine is SBR 5" TROS 3 lug barrel with SWR Trident 9 silencer.  It has the RRA 9mm hammer, 9mm bolt (not ramped), heavy buffer, dedicated Hahn's  block, and KNS Gen II pins.  The ammo used is homegrown..  147GR, with 3.4 gr bulleyes to stay subsonic.

Any clues as to what is causing it?

Have you lost either the detent or the spring for the safety selector?  Do you have an ambi. selector that is not installed tightly enough?

Disconnector is hitting on the selector. I would pull the selector to see if there is some extra on it. Look at the end of the disconnecter and make sure its not too long.

I don't think its the detent pin or the spring, they are all intact and present.

I will take a look at the disconnector and selector this evening.

Thanks,

I had the same experience with my 9mm sbr this past week too and thought it was the disconnecter. But, when I got home and disassembled the carbine here's what I found.

The pin that holds the weight in the bolt had fallen out during my last cleaning (found the pin on my work bench). The missing pin allowed the weight to rotate in the bolt which would not allow the hammer to fully reset, a condition similar to having a disco that is too long.

check to make sure you don't have a blown primer or other debris stuck down in there either.