Hey Guys ,
   Need your help in troubleshooting a colt 20" HBAR target post ban. It fires the first round and ejects it and loads the second round but does not recock the hammer. And sometimes puts a small indent in the next round but does not fire it.Any clues?

 Help much needed.
  Title24u
Also the lower is completely rebuilt and it has a new bolt and carrier.

get a new trigger group. Possibly sear worn.

need more info.
take the upper off.
cock the hammer.
pull the trigger but do not let the hammer hit the frame.
keep the trigger pulled to the rear.
cock the hammer all the way to the disconnector.
release the trigger (let it come forward)
does the hammer catch or fall all the way forward?

you may need new springs and a trigger group.

On the second round, the hammer is being cocked, just not being held and is following the carrier back forwards.

Since you state that the lower is fresh built, Then pull the trigger and change the direction of the disconnector spring. Chances are that yours in backwards or missing.

The large side of the spring goes down into the trigger back channel which allows the trigger to retain the spring, and the spring to have full movement and function.

When your done with the disconnector spring,  Re-do your FCG function test, including testing the disconnector under full load.

Hope this helps.

Is this a stock trigger or an adjustable single or two stage?  If it's an adjustable two stage or single stage, it could be the sear disconnect is too light or the overtravel needs to be increased.  If you hold the trigger back will cocking the hammer, after you have the hammer locked back, release the trigger.  It should move to the forward position and make a clickong sound as everything locks into place.

Pull out the disconector, shave .030" off the underside of the front foot.  Problem solved.