The high nitrogen ratio in veg is less acidic than the high phosphate ratio in flower. Plant available phosphate (H2PO4) is only a little less acidic than the pH down phosphoric acid (H3PO4) you’re using.

pH is the issue. Measuring pH could be another issue. Digital meters lose their ability to give accurate readings over time as the neutral reference probe that contains potassium chloride will leach out giving false readings. I always suggest having pH drops as an analog way of checking pH. Get the pH down to 5.5 and you’ll be good.

Cannabis plants can reach sexual maturity at 4-5 weeks old. Doesn’t mean you’ll ever see flowers in a month. The transition to flower time is 100% dependent on the photoperiod/s it was crossed with. After a plant has reached sexual maturity it will still take several weeks to begin showing early bud sets.

Is there any media under those rocks or is it 100% hydro? If so, what’s the water temperature? About 68-70F is where you want the water temperature with hydro.