How can I save her ? Been having Soil PH issues. Northern Lights Photo 9 week veg

My situation :

9 weeks veg in soil, temp around 26 Celsius 72% rh

Noticed when I gave 6.5 ph in my runoff was 5.5 ph.

I flushed yesterday ( 2.5 l in , 650ml out ) the last run off ph was still only 5.6

Should I flush again ? Can I still resolve the issues ?

Any help is much appreciated thank you 🙏🏽