The part that doesn't add up for me is when he says it light strikes when running but if manually cycled it fires ok.
If it cycles ok but just doesn't fire I think that eliminates springs, buffers, gas, etc as it is able to eject and reload, ie not short stroking..
Given it fires ok the first time or if manually cycled I don't see how thats a trigger or firing pin problem.
To me sounds like the firing pin is ok but isn't making it far enough, possibly bolt/lugs/carrier/chamber off somehow. Loose barrel could be in the ballpark too. Sorry for the repeat but wanted to add some color to it - remove the extractor and ejector, reassemble the BCG minus those parts, and perform a headspace check. Note how the end of the BC aligns with the upper when fully closed too (this is a loose tolerance but if something is F'd it will hang out or way in when shut)..
If thats all ok, put you ejector and extractor back in there, verify that at a 45 degree angle you can drop the bcg down the upper and witness it engage typically. A smooth running AR works this way, if something is off or sharp, needs a little wearing in, such will generally not work - grease lugs, try 100 cycles by hand via the charging handle, and repeat the test.