Root growth severly stunted

My girls are half way through flower and there roots are nothing more than pathetic.
They get new growth all the time then 3-4 days later it withers into brown string then into nothing...

I've got them in a drip hydro system, 3" net pots with hydroton. I water for 3 minutes every 3 hours (1.1 gal for each plant per watering). [pump is 2.6 GPM * 3 minutes / 6 plants]

As far as I can tell everything is light proof, I'm at ~850 ppm with technaflora nutes (using 3/5 recommended dose). PH is 6.2 consistent. using store bought RO water.

Roots tend to dry out and die off if they cant get enough water.
Try more frequent feeds like 15 mins every hour.
Just have to get the right feed setting/run times and you should be fine
Good luck

Alright I set my pump to water for 15 minutes every other hour, my timer only has 12 on-off cycles to set so that the best I could do, still its a lot more than before. I hope this helps my girls, thanks.