Vertical grow uneven growth

I've had a strange problem with my vertical op. It's basically three rows with three separate bins. 2 rows looks like a dense jungle, one row is filled with stunted plants who barely grew past week 4.

I change the recipe in all three bins at once and nothing was changed in row 1's recipe...the same stuff in all three bins. The pump didn't appear to fail, as I keep a close eye on how the gutters drip water back in the bin and I see no diff between the bins. The pumps are old and the flow may have diminished in intensity, but after 5 mins on all my rockwool bricks are leaking from the bottom, so I always figured a diminished flow speed from a warn out pump might be a non issue...

Aside from that, same light, temp, humidity in all three rows. No mildew, no pests...no event where I notice the plants in that row just go limp like when a pump burns out and misses more than a few waterings. My on\off cycle is 5 min on 45 off.

Another slight difference, the ph went down to 5.5 on that bin compared to a mean average of 5.7 on the other two. I just don't know if that last detail is the cause or symptom of slower, stunted growth.

Thanks for any ideas...

On another note, the affected row has a few older leaves that yellow but that aside, no discoloration, same dark green leaves than on the other two rows...

post some pics

Healthy row 1....

Healthy row 2 ...
Crappy row....crappy row close up...

Crappy row close up 2

To summarize: 2 healthy rows, one row with smaller, more stunted plants. Same leaf color in all 3 rows; dark, healthy green. No leaf tips burnt, some of the older, lower leaves in puny row tend to yellow but those are very few.

PS...sorry for orange eye rape..