sounds like those nutes are not ph-buffered, even if ph-balanced to start, because they clearly are drifting. Good hydro nutes should be resistant to drift. It's a common sense thing for a manufacturer to do.
are there symptoms on the plantt? if not, don't overreact too much. Keep ph-down whatever you add.. you can try to go slightly lower pH but risk more complications with more drastic fluctuation, so proceed with caution.
Just keep fighting the good fight. Ph-adjust and keep an eye on it -- most importantly the health of the plants.. if they are okay, don't do anything drastic.
Consider a different product. E.G. I can tell you the pH of Jacks 3-2-1 will not drift on you. I'm sure there are others, too. Fertilizer is a commodity. There's really not a quality difference from one brand to the next, so don'toverpay for it. If you buy jacks in 25lb dry bulk bags, you'll save a shit load of money too. I grow about 70-80sq ft of garden over the course of a year. I spend 30 dollars/year on fertilizer. That's like 5 full 4x4 tents or 3 full 5x5 tents per year for better comparison for most people... i bet most spend a lot more than 30 USD on fertilizer if buying marijana branded stuff with cartoon labels that look like they belong in a sugary cereal aisle at the supermarket.
plus, their formula is superior too. it'll work with a wider range of genetics. I rarely see leaf symptoms no matter how many strains i grow concurrently and pull off the same 55-gallon reservoir of fertigation water. pH is rock solid.. so rock solid i don't even spot check anymore. I can just assume it's fine, because it is.