I took my newest build out to the range today for testing.

I've pieced together this rifle over the past couple of months due to parts availability

Lower is a Stag with PSA lower parts kit.  Upper is a BCM

About 1 out of every 8 or 10 shots, the trigger will fail to reset.  If I flip the safety selector to the safe position, the trigger will reset.

I've never experienced anything like this.  The hammer spring, disconnector spring, and trigger springs are installed correctly.  What could cause this? An out of spec hammer or disconnector possibly?

I'm not 100% sure..... but what weight buffer are you running?  Have you tried a heavier one?  I think, and could be wrong, but is this bolt bounce?  I've never experienced it first hand myself.  Dis-connector spring installed correctly?

Don't know buffer weight.  I bought the PSA MOE complete lowere build kit.

All parts are appropriately lubed.  The strange this is that I cannot replicate this problem at home.  I've dry fired the gun 20 or 30 times, kept the trigger held in, cycled the bolt carrier, and I always get a positive reset when I release the trigger.

I seperated the upper and lower and brought the hammer back til it caught on the disconnector while keeping the trigger depressed.  When I release the trigger, the hammer is released and catches on the next "step"

I've got an ALG combat trigger on order, but would like to fix the issue in the meantime.  I don't see or feel any burs on the disconnector.  I suppose it could be out of spec.

Someone on another forum mentioned short stroking due to a loose gas key.  I could not find a complete bolt carrier group, so I built a carrier using a Fulon Armory stripped carrier, BCM gas key and screw, MA bolt, cam pin, firing pin, fprp.  I staked the gas key with a MOACK tool.  I would think if that were the issue, I would have more problems than a failure to reset the trigger every 8 or 10 times.

Where would you begin? I have an ALG trigger spring, disconnector spring, and hammer spring I could try.  I don't really want to get into filing down the disconnector.

Something else I forgot to mention was that the MOE grip was way too tight. I had to remove some material to get it to fit on the receiver. I don't think the safety selector spring is putting enough tension on the detent.  When I flip the selector to the safe position, I don't get a positive click and the selector does not rotate all the way down.  Is it possible that this is allowing the safety selector to move under recoil and affecting reset?

What pistolgrip screw did you use, and did you use the external tooth lockwasher on the pistolgrip screw?

It's real tempting to use the long standard allen head pistolgrip screw instead of the short pan (flat) slot head screw (with locktite spead on it) that is supplied with MagPul pistolgrips.  The short screw is a PITA to install as it keeps falling off your screwdriver.

BUT, if you use the longer, easier to install allen head screw, you will cause functional problems with the FCG.  The mounting surface of the MagPul pistolgrips are thinner than stock A2 and A1 pistolgrips.  Installing a MagPul pistolgrip with the standard length allen screw, and the screw will protrude too far through the lower receiver, and hit the bottom of the trigger, pushing it up against the selector.  This results in a trigger that will not reset sometimes, and/or a trigger that will not reset without flipping the selector sometimes.

The answer is to go to your local hardware store and buy a pan head allen screw that is the same length as the MagPul short pan head slot screw, or remove the locktite from the MagPul screw/bolt, use the external tooth lockwasher, and suffer trying to install the MagPul screw.

I used the screw that came with the grip.

I just removed the FCG and cleaned it thoroughly and relubed it.  I replaced the trigger,hammer,and disconnector springs.  The psa disconnector spring was a little taller than the one I replaced it with. The new hammer spring was a lot stiffer than the psa one as well. I found an ejector spring and used it to replace the safety selector spring.

When I perform a function check the reset is far more positive. I did not find any burs on the hammer or disconnector. If this doesn't fix the issue I'll replace the FCG.

Selector moves freely between safe and fire.  There is no takeup with the trigger when the safety is engaged.

After removing the grip, I screwed the pistol grip screw into the receiver to see if it protruded into the trigger area.  It stops well before the end ot the threads in the receiver.

Would too light of a hammer spring or disconnector spring affect reset?  Both of the PSA springs seemed really flimsy.

ETA that the trigger spun freely around the trigger pin when out and the trigger itself fit freely within the walls of the lower receiver

I have seen triggers "Stick" due to it fitting too tight in the reciever. If all else fails, take everything out. Put the trigger in the reciever and see if it moves freely. Then take that out and do the same with the hammer.may have to file a few strokes off the side of the trigger or hammer. Sometimes it just the finishing got alittle too thick.

It only catches on the second step if the trigger is held back

Just got back from the range and happy to report the issue is resolved.

Either the disconnector, trigger, or hammer spring had to be out of spec.  I simply replaced all 3 and now its GTG.

Glad you got it running - even if you used the auto shop mechanic approach!!LOL!!! Know it was frustrating.