PH drifting below 4, by middle of flower. Soilless(Agro A5-HP), watered when dry

Left over nutrients in the media are acidic, so when you feed, they get left in the media and then this dries in the media. When you water again, it re-solubles the nutrients , lowers the PH of the water and your runoff, while redepositing more nutrients into the media to start the process all over again. A solid flush will remove the dried nutrients and allow you to place more nutrients into the system at the proper PH.