At the range yesterday, my Colt MT6721 with JP Trigger failed to reset 4 times in a row during my shooting session.  The failures came on fired rounds #12,13,14, and 15.  I broke the weapon down each time and checked for any obvious problems.  Finding nothing each time, I put the weapon back together and fired again.  Each time, my AR cycled normally, but the trigger failed to reset.  

As mentioned, this happened 4 times in a row.  The trigger always reset normally when I cycled the action manually.   The trigger was installed by a reputable gunsmith about two years ago.  Since that time, I have never had a problem with any of the 1000 or so rounds I have fired.

When I got home, I broke the gun down any sprayed the heck out of the trigger group with aerosol Gun Scrubber and reapplied Miltec oil.  The rifles seemed to pass all function checks, but it never failed any of them at the range either (except for live firing).

This is my primary rifle.  I am concerned about its reliability.  Any advice would be greatly appreciated.  Thanks.

I believe the hammer was back (I don't remember for sure, but everything looked OK when I opened it up), but the trigger still failed to reset.  I don't know a lot about the inner workings of the trigger group, but I am puzzled about the fact that the trigger always reset by cycling the action manually, but not on four rounds in a row of live fire.

Sounds like too much disconnector to hammer engagement.  Is this adjustable on your trigger assembly?

Sounds like your trigger was working ok and then this developed. The first thing I'd check would be for any  primer anvils or cups floating around in the lower. They can cause some weird malfs. I've seen it happen several times over the years. You may need to remove the hammer and trigger to do a complete check. The anvils are small and like to hide under the trigger.

Concur with the suggestion of checking for debris below the trigger.  Also, the JP trigger does need periodic fine tuning, check the sear and disconnector engagement screws.  With the disconnector engagement, too little it will doubled or follow, too much it won't release.  Place a call to JP with your AR symptoms & their tech guys should be able to help you directly.