Work on CO2 and make sure you dose the KNO3.
It's nothign deep here, just do the work, maintain the CO2 brew often, keep the CO2 at 25-30ppm and check the pH at night before the lights go off to see what the CO2 is.
Swap bottles(use 2 and alternate weeks). Do large weekly water changes, dose nutrient back and then 2-3x during the week.
Regards,
Tom Barr