Ok, I've got a RRA 9mm carbine that I built and have run approx. 500 rounds through with no problems.  Yesterday I was shooting it again and after about 100 rnds the trigger & hammer quit resetting properly.  A new round would be loaded but the trigger would not reset.  If I manually chamber another round it will fire ok but again, the trigger won't reset for the next round.  I've tried various things but can't get it to work properly.  Please help!

Not to state the obvious, but did you take out the trigger spring to make sure it did not break?  Maybe take the FCG apart and clean.  It is possible that either the spring broke or some crud is binding the trigger.  

Are you saying that the hammer is following the bolt? Or that the disconnector it not releasing the hammer?

Ok, some more info.  Yes, I did completely dis-assemble the FCG, inspect, clean, and re-assemble.  I tried shooting some more and had better but not perfect results.  I noticed that if I place my trigger finger behind the trigger and slightly nudge it, it will "click" forward and the gun would then fire if I pulled the trigger.  Now about 50% of the time it resets correctly and the other 50% I have to nudge the trigger forward.  Any more ideas guys?  Thans for your help so far.  

That sounds a lot like a bad trigger spring.  Not that it may be broken, but maybe it's gotten weakened.  I'd try a brand new spring and see whether that fixes it.

Hammer spring gets installed with the closed section on the back side (butt stock side, just below the disconnector engagment sear hook) of the hammer, and the spring legs go on top of the trigger pin.

The trigger spring installs with the closed section under the front section of the trigger, and the legs forward resting on the bottom of the receiver cavity.

Also helps to make sure that you have installed the disconnector spring large coil-side down into the rear trigger slot, and make sure that all the pins/parts are lubed with CLP before installing.

To end, when you have the hammer back out to lube, check the J pin to make sure it is peened into the hammer.  A loose J pin can walk out and catch the front of the receiver cavity and bind up the re-set.

Quoted: That sounds a lot like a bad trigger spring.  Not that it may be broken, but maybe it's gotten weakened.  I'd try a brand new spring and see whether that fixes it.

Give that man a ceegar!  I replaced the trigger spring (and hammer spring for good measure) and just ran 60 rounds through the gun without a hitch.   Problem solved.  Thanks to those that replied.