Took the new rifle to the range to test fire it and maybe get the scope zeroed. It wears a mid-length SOTA upper with a nickel/boron BCG, 16" barrel. Anderson lower, PSA polished trigger group. Lucas Gun oil, per instructions found on this site. Ammo is 5.56 62 gr LAP that runs just fine in my PSA rifle.

I single-loaded four rounds, chambering each round from a 30-round Magpul magazine. Rounds fired and ejected properly, and the bolt locked back. So far, so good. I then loaded a full magazine with the same ammunition and chambered the top round exactly as I had the single rounds. The trigger would not budge. I ejected that round and tried again, with the same result. I repeated the process 8 times, and the rifle never would fire. Ejected rounds showed very light firing pin marks on the primers. That concerned me, so I wrapped up and left the range.

At home, I pulled the BCG and stripped it, but couldn't find anything wrong. I checked the chamber, the trigger group, and the bore. Nothing jumped out at me. I reassembled the rifle and ran some snap caps through it. They  chambered just fine and the trigger dropped the hammer the way it should.

So... it looks like my firing pin was sticking, but I couldn't find anything wrong in the BCG. I assembled the lower from mostly PSA parts. It passed all the safety checks. I am baffled. What say you more experienced folks?

was the safety on?

Sounds like a popped primer fell under your trigger locking it forward, meanwhile the light dimples were from the inertial firing pin tapping the primers as usual upon bolt closing.  If so, could be bad luck or a tight chamber.  Green tip is often hot ammo which increases the odds.  I'd reinspect your trigger group cavity for a primer or consider if there has been a chance for it to fall out.  The trigger not budging after normal prior function is pretty indicative.

Stray primer is my bet also. What's even worse than a primer in the trigger group is getting one into the cam pin slot. I was really worried that day. I didn't think I would ever get the gun apart. I had some surplus Malaysian stuff that was loaded hot. 2 or 3 strays per session. 

snip

Check your empty brass for missing primers.

Shotgun the rifle open and with your thumb on the hammer pull the trigger making sure the hammer doesn't smack the receiver. The hammer should swing freely when you pull the trigger.

If not, you'll need to remove the trigger group and find out why. As already stated you may have ammo that's too hot for your rifle and it's blowing primers. One got stuck in the trigger works. You may have installed the hammer pin backwards and it doesn't provide enough pressure to reliably drive the firing pin.

If the trigger wouldn't move, you had the safety on as already stated.
If we just are having trouble with our verbiage, and the trigger actually did move but the rifle didn't fire, it would indicate an out of battery situation - possibly the round from a loaded mag had too much drag and stopped the bolt.
In a case like this, just for future reference, never try to force a gun to fire when it doesn't want to. Stop trying to fire, make the gun safe and try to figure out what's going on. Every 'Ka-Boom' story I have seen always starts the same way - 'it wouldn't fire so I kept trying, and...'.
Is there any chance that there's part of a shell in the chamber that you have missed? Try the 'plunk' test - remove the bolt carrier and drop a round into the chamber to see if it easily loads properly. Don't forget, the case head and rim will protrude from the chamber normally.
Except for the theory already stated of a primer or other piece of brass jamming the trigger group, that's about all I can think of right now.

I finally got time to dissect the rifle today, and sure enough, big as life (or maybe almost as big as a lentil), there's a stray primer rolling around in the trigger housing. That being said, the "plunk" test was performed with 5 different rounds, and they "chambered" normally.

I didn't recover my empties at the range, mostly because I had no idea where they'd flown and it hadn't occurred to me in the first place. Good tip for next time, though, thanks! I can guarantee the safety was not on SAFE, it was the first thing I checked when the trigger went on strike.

So, my LAP gives me a 25% failure rate, and I still have about 500 (well, 496) of the little bastages. Not best pleased about that, but I appreciate the guidance.

popped primers are not that common.  Ive had AR's since 2008 and have thousands of rounds in that time between my reloads and factory and have never had a popped primer.
Ive been shooting since the 80's and haven't had one, get into my action.

I made a range trip today. The rifle functions perfectly; not only does it fire, it actually SHOOTS. It's capable of 1 MOA out to 200 yards. For an unfamiliar rifle with a 16" barrel and a $30 aftermarket trigger, and in a 20 MPH crosswind, I'll take that all day.

Thanks to everyone who responded to my queries!

 got a vid  of that?

1 MOA out to 200 yards with a 20 MPH crosswind, you should be shooting professionally, check that. You should go to any AR manufacturer you desire, get the CEO to take you out to their firing range and show him. They will pay you millions for you to shoot competitions and wear their shirts, caps and they will give you as many AR's you want and instantly make you an executive officer.