About 3 weeks ago I purchased a Colt 6960 CCU.  About the same time LaRue put their triggers on sale so I ordered two.  I had previously purchased and installed 3 MBTs with no issues.

Right off the bat I made the mistake of not shooting the rifle before installing the new trigger.  I should not have done that.  In any event,  the installation went smoothly until I tried putting the ambi-safety back in.  It didn't want to go in, but  I eventually got it in and did the function test.  All appeared good--trigger released on F and would not release on S. I did the function test several times.  So I reinstalled the detent, spring and grip and went off the range.

The first 2-3 shots all was well, but after that the rifle effectively became a single shot.  After the first round, the trigger appears to be stuck.  However, by moving the the safety to S you can hear a click.  Then when I move the safety back to F, the trigger works and the round fires.  I have to repeat the process for each shot.  I tried backing out the grip screw by several turns just in case the tip of the screw was causing a bind, but that did not make a difference.

Suggestions?

Aftermarket ambi safety?  Sounds like there's a clearance issue between the safety and the tail of the trigger shoe.  
You might look and make sure there isn't some debris in between those two surfaces.  

Also check and make sure the trigger springs are installed correctly.  Even if the legs are facing the correct direction, is the square part of the coil on the correct side of the trigger and hammer?  

I'd swap in a stock mil-spec safety and see if that takes care of the problem.  Otherwise, call up LaRue, I'm sure they'll make it right.  

Could be an out-of-spec lower, too.  But with Colt, that is probably less likely than a lot of other brands.

Who makes the ambi safety?

The 6960 CCU comes standard with an ambi-safety, which I assume is made by Colt.  I will check the trigger spring location.  In fact, I think I will take the trigger out tomorrow and start over.  I probably have a single sided safety in a box somewhere.  I don't really need the ambi-safety.

You may have to do some filing on the safety drum.

The tail of some aftermarket triggers is wider than stock.

The notch in the drum is causing the trigger tail to hang up.

With the lower shouldered, look down into the fcg pocket.  Usually the right side of the drum notch needs to be widened a tiny bit.  You can use needle files or a dremel cutting wheel.  You don't need to remove much.

I installed a Geissele high speed that required me to remove a tiny bit from a BAD ambi safety drum.  In my case, I could barely feel the trigger tail hitting the safety.

Thank you, Rob78.  The fact the safety did not want to go in is probably a good indication that your diagnosis of the problem is correct.  If I can find my spare single sided safety, I will try it first and see if it goes in smoothly.  If it does, that may fix it.  If not, I will take it out and start over.  I don't have a dremel but I do have a bunch of needle files.  

BTW, none of the other lowers (Ruger, Aero, Bushmaster) had an ambi-safety and the LaRue trigger went in with no issues and all the rifles fire reliably.

Did I fix it?  I probably won't have an opportunity to shoot for several days or longer, but I will report the outcome no matter when my next shooting day is.

Here is what I did.  I carefully compared an earlier (and successful) trigger install in an Aero lower to the Colt lower. (Should have thought of that earlier.)  It appears that the coils of hammer spring on my Colt lower were facing in the opposite direction from that of the Aero lower.  So I removed the trigger, hammer, and safety and flipped the hammer spring.  I re-installed the trigger and replaced the ambi-safety with a single sided safety that I was able to locate without too much trouble.  It went in with no drama.  Tested the function of trigger and safety several times and all appears to be good.  But the proof will be in the shooting.

Thanks for the pointers, guys.