Buy a lux meter, all plants need a good amount of light to grow high quality bud, a group picture would have helped but my guess is the nl was on one far side and the s.c was the other, the 2 centre plants grow taller than the side plants, blocking out some of the light they where getting, the par level was too low resulting in a very low crop for the 2 plants.
If the led had a good light spread, using the meter you could have swapped the 2 center plants over 3 weeks in flower, adjusted the height of the led so the side plants where getting the most light they could from the led, center 2 plants could be 6-12" shorter, but the led gives out more light directly below so it would all balance out.
Lux meter are cheap, could even use the app, the amont of lux is less important, anywhere between 30,000 and 60,000 should be OK, over 40,000 would give you better results