Pull the FCG, clean all the parts and check the receiver cavity for  a piece of blown primer that found it way under the trigger, as well as maybe some debries that may be sitting under the front of the disco and not allowing the disco to fully seat all the way forward in the trigger.

When you put the FCG back in, make sure to lube all the the parts, make sure that all the parts are moving freely (disco), and that you do a function check on the rig to make sure that the disco is capturing the hammer at not only the rear of carrier stroke with the trigger held back, but retaining the hammer on the disco as the carrier slams home.


Also, if you installed a over travel  adjustment screw, make sure that the trigger moves at least 1/8" past hammer release on a the full trigger pull.

Two stage trigger,


So as stated, you either have something under the bottom of the trigger that is not allowing the trigger to be pulled all the way back for the disco to work correctly,

Or

The disco is not free moving in the trigger itself , or have crap under the front of its front toe, and the disco sear is not coming all the way back forward to catch the hammer with the trigger pulled all the way back,

Or,

You have some sort of trigger over travel installed in the FCG that is  not allowing the trigger to be pulled far enough after hammer release to get the disco sear in the correct position to catch and hold the trigger until trigger release as the carrier slams home.


So to be blunt, if the bolt is coming back far enough to strip a round out of the mag, then hammer is being cocked back far enough for the disco to catch the hammer, but something is going on that is not allowing the disco to retain the hammer with the trigger head back as the carrier is slamming home.


So on that note, pull the disco part on the trigger and first dry fit the pin to it to make sure it not binding on the pin. Next, reinstall the disco with the pin and spring outside the rifle, and then make sure that the disco is not binding up on a bur in the trigger slot as you work the disco by hand.

Next, install the FCG back in the rig with all the parts CLP lubed, including the pins, and make sure that the sides of the trigger are not binding up against the sides of the receiver cavity void or the tail of the trigger binding up against the side of the U slot in the selector, and when you pull the trigger back and release it, the trigger snaps back all the way back forward on its own.   From here, hold the trigger back, over cock the hammer so the disco snaps onto the rear trigger hook, put your hand in front of the trigger, and slowly release the trigger.  The rear hook should release the hammer and the front hook should grab the hammer.

The last test is to hold the trigger back and hold it back (can tape it all the way back if you need and extra hand) with the rifle back together, pull the charging handle and lock the bolt on the catch, hit the bolt release to allow the B/C to crash back home with the trigger still held back, then release the trigger and pull it again.   If you are not getting the hammer to release on the trigger pull after release (disco let loose of the hammer on the B/C crash), then the disco is binding in the trigger shoe/disco spring is not installed. the trigger is not able to be pulled far enough back,  or you have the disco spring installed the wrong way.