Recently finished a lightweight build with a LaRue 16" PredatAR barrel and Anderson lower.  LPK is either an Anderson or PSA, and BCG is a PSA premium (everything is new except the gas block).  The issue I am having is the trigger is not resetting after firing.  Bolt/carrier is cycling, pulling next round from the magazine, but trigger wants to stay in the rear position.  Flipping on/off safety resets it, as does a slight push forward on the trigger.  Hammer and trigger springs are installed correctly, and disconnect can be pushed up and down with the upper off.  I can do the same thing with the upper removed, by holding the hammer back and forcing the disconnect to hold the hammer, and it wants to still catch even with the trigger released (although not as consistent as with the upper installed).

Is this likely just a FCG issue, or something else?  I have a 2nd FCG I can try in the lower, but wanted to check here before I start throwing parts at it.

Having the same problem with my trigger I used an Anderson lower parts kit. I installed everything correctly but trigger is not resetting and I have to manually reset it most of the time. I changed out the hammer spring and still same problem so it could be the design of the hammer since it's not a standard style.

Check every contact surface and pin for burs.

This will happen if the receiver is too deep or the trigger spring too weak,

This, I've even had Anderson trigger groups that were actually crudded up and and had to be scrubbed good with a brush and solvent. Some type of packing grease that dress out or something.

Trigger spring on wrong.  Google it up. 

Not in my case I removed the fcg from two of my rifles. One was factory gun both had springs in same orientation. That why I stated that it could be the hammer design since it's non-standard

A friend's new PSA lower did this recently, I just lubed it and cycled it 50x and it started resetting.

If that doesn't work take out the FCG, clean up contact points and carefully reinstall

Or upgrade to a MBT

Are you 100% sure  you installed the hammer spring correctly?  I would check that before anything else.

If it is an Anderson I would be checking for burrs on on the hammer hook, seems to have been a problem for them as some of their hooks seem to have a ridge of sorts just beyond the edge of the hook.  The issue is easily corrected.

Did you apply oil or grease?  Contact surfaces on the FCG need grease, not regular lube or oil.

AeroShell grease #6, Mobil #28, or equivalent are good choices.  A good automotive wheel bearing grease will do if you don't have access to those.

If grease does not solve the problem, you'll probably need to contact Anderson.

Is disconnector spring upside down?

Anderson and PSA disconnector sprinfs are known to be cut short.  Replace and it is fixed.

I don't think so, but I will check it once I can tear the lower down later.  I believe I have a couple spare disconnector springs, so I'll try them out.

This happened to me on my first build. Trigger spring was on upside down. It would fire, cycle, but no trigger reset.

I've had this happen with several Anderson lpks. On mine after about 2 mags of having to push the trigger forward it would start working. On one I decided to bend the trigger spring legs down to give it a little more preload. That worked. I think it is just that the contact surface between the hammer and disconnect or is a little rough. It has always smoothed out after a few shots for me but bending the trigger spring was an instant fix.

This right here.

Finally had a chance to play around with the lower today, and pulled the FCG.  Couldn't find anything obviously wrong with any of the components, but I went ahead and cleaned everything and reinstalled....still not resetting consistently.  So, I tried an extra FCG I had, and it still did the same thing.  All springs were correct, including the disconnector spring, so I was a little concerned that maybe something in the lower itself was out of spec.  Tried a 3rd FCG I had, and it seems to work fine.  I haven't had a chance to actually fire it, but it is resetting with dryfiring and cycling the BCG.  

Not sure what the issue is with the other two FCG's, so I'll have to try them in a couple other lowers and see if the problem persists.