I am experiencing this problem lately, it started couple weeks ago first was every once while now it happens every 4 to 5 rounds. I've already replaced bolt catch, catch spring assembly, the problem persist. happens to every upper,  any ideas?? thanks!

colt factory m16 lower, colt usgi full autofcg.

Have you tried different magazines?

If the BCG is locking back it's because something is pushing the bolt catch up and Into the path of the BCG.

I have the opposite problem, my BCG doesn't lock back on the last shot, but it's because I shoot suppressored and the internal parts get gummed up.

I did, at first it will only happen on magpul d60, now happens on every type of mags.  I've also strip down the lower and ultra sonic the lpk.

using different BCGs on each upper or sharing the same one?

Also try shooting it with one round in the chamber and no Mag inserted. If in still locks back the. It's not a Mag issue. Being a colt it shouldn't be out of spec but ya never know.

How do you hold the rifle? You could be applying pressure to the bolt
Catch with your hand.

it won't lock back in semi, only occur when running fa.  
i did some testing, seems like this symptom occur more frequently on higher cyclic rate uppers and suppressed uppers.

Suppressed uppers tend to be over gassed but Im not 100% sure how being over gassed would translate into the bolt catch causing problems.

Only thing I can think of is the action is so violent that it would cause the bolt catch to bounce up and stop the BCG. I feel like that's a stretch as far as possibilities go.

I know you said you cleaned everything already but I would strip the upper one more time and reassemble. In particular make sure that the notch for the bolt catch is cleaned and make sure you are installing the correct spring.

Beyond that Im not sure what would be causing the issue. If that doesn't resolve the issue you will probably have to sent it off to a gun smith to be worked on.

As mentioned above, you don't grip the magazine well with your support hand when firing, do you? If you do, you might be inadvertently engaging the bolt catch by pushing inward on the protrusion on the bottom of the bolt catch. "It wouldn't be the first time..."

I'd say this. I don't own an f/a rifle but a bad or wrong spring would be my first guess.

Its hard to tell 100% what may be causing this but if you have already replace the bolt hold open lever, the bolt hold open spring, the issue only happens in full auto you may want to consider replacing the magazine latch.

You mentioned that it first started happening with a Magpul D60 magazine.  Its possible that the heavier drum in some way monkey-ed up your mag catch which is now allowing magazines to move up & down more in the magwell than they should.

Quite a few magazines, (especially the newer polymer ones) dont have a lot of room in the cutout where the LRBHO actuator tab is located.  

Its possible that during full auto fire the mag is essentially slightly bouncing up and down inside the magwell and the body of the magazine (where the cutout for the follower tab is) is actually coming up and making contact with the LRBHO lever resulting in the bolt being held back.

I would take a magazine that more reliably creates the malfunction and see if I can get it to touch the LRBHO lever by manually manipulating the locked magazine  up/down/left/right with the upper removed.  Even if the mag body even just touches the LRBHO (and may not be enough to get it up into the path of the carrier)...there may be enough force during actual firing for the mag body to strike the LRBHO level and bounce the lever up into the path of the carrier hanging it up.

Just something to consider before potentially handing over your $25K receiver to a gunsmith.

I do not hold the mag or magwell while firing, most of my uppers are setup with vertical grip or angle grip.  

thanks for the inputs, i will strip down the lower again this week replace all the springs and sacrifices a mag by cutting the follower and machine the lip down making sure mag wasn't causing this.  will report again after range trip.

When you replaced the bolt catch spring, did you make sure that the bolt catch detent was present? The detent goes between the spring and the bolt catch.

yes, i did remember.