Could of things could be the problem.

Blow primer under the trigger,
Wrong spring used for the disco, or the correct spring installed up side down (large coil side should be downward in the trigger slot under the disco tail).
.174" channel FCG used on .154" trigger pins.
Disco binding on the trigger pin/in the trigger slot, or crap under the front of the disco that is holding the front of it off the top front of the trigger.
Disco is just out of tune.



Lets start with pulling the FCG out of the lower receiver.

Start off by looking at the bottom of the lower receiver, and make sure that there are not blow primer parts (love to hide at the channel for the grip screw).

Trigger in hand, make sure that the correct disco spring is installed and that the spring is installed large coil side down into the trigger slot (have to press the spring into the slot, so if the spring is falling out, it's not the right spring, or installed upside down).

Now lets install just the trigger pin into the trigger and make sure that the trigger can freely rotate on the pin.  If there is binding, then check the sides of the C channel on  the trigger for milling burs, and if needed, remove then with a small jewelers fine.

Next, pull the pin out of the trigger, and install  the pin on just the disco.  The disco channel should fit the pin tightly, but the disco should move freely on the pin.  If the side channels have burs, then use a counter sink to remove the side of channel burs.

Now install the disco on just the trigger with the trigger pin/disco spring, and make sure that disco can move without binding, and has spring tension pushing the front of the  disco down tight against the front of the trigger top surface.  If you have binding, find the burs (may be on the inner slot of the trigger, so use the small file again and remove the burs.

Now before we reinstall the FCG, install just the pistol grip with grip bolt and washer, and make sure that the tip of the grip bolt is not protruding up into the bottom of the lower receiver void.  If needed, shorten the tip of the bolt, and again, make sure that you have the grip bolt washer in play!!!

Now reinstall the FCG back in the lower receiver, making sure to CLP lube the pins, the parts/receiver channels and the parts of the FCG as you are reinstall the FCG back in the lower receiver.

Now lets check the release timing of the disco, and if needed, retime it.

Lower receiver only, hold the trigger back, cock the hammer back so the disco is retaining the hammer, then with your hand in front of the hammer so it does not crash into the front wall of the lower receiver, slowly release the trigger and see if the trigger catches the hammer once it released.  Here, but guess is that the hammer missed the front of the trigger on the disc release, and the hammer slams into your hand protecting the front of the receiver instead.
Also note, we do not what the disco to release the hammer until the trigger is dam near all the way home in the forward position (read trigger has less than 1/16" before it stops moving forward).

Without touching the trigger, cock the hammer back until the back hammer sear gets as close to the disco hook sear as you can to see what the free gap between the two looks like.  The back hammer sear should miss the tip of the disc sear by .001 to .003" (thickness of a sheet of paper).

In the below photo, the Free gap between the disco and hammer back sear is huge, and the disco needs to be timed.


As for how to re-time a disco, you will remove metal from the front/bottom of the disco where it seats against the front/top of the trigger.


Work slowly with a few dry fittings along the way as you are filing off metal from the front.bottom of the disc, and what you want is to have the gap between the two with the trigger untouched so the two just just every so lightly kiss as you are over cocking the hammer past the sear.



Now go back to testing,  Hold hold the trigger back, cock the hammer to the disco, and marvel on as you slowly release the trigger, not only does the disco release the trigger just a hair before the trigger is all the way forward (which will prevent doubling when you are even shooting at the bench), but the trigger catches the hammer each and every trigger reset  as well (no matter how slowly you release the trigger).