Hydro is a constant battle IMO. I started with hydro but now do coco exclusively cause its close to hydro but without the res issues.To answer your question tho, it could be many things.
First off do you use tap or RO/distilled water? RO water is hard to keep stable because there is no minerals to act as a ph buffer so i found that straight RO water fluctuated drastically. On the flip side, sometimes tap water is impossible to bring ph down i have found. I had to switch to RO/tap water mix as straight tap water would not go lower than 7. Almost like something the city started adding to the water was making it extremely difficult near impossible to keep my res in the proper range.
Second, are you measuring ppms? i found it imperative to measure ppms doing hydro cause it would help me diagnose issues. For example, i would measure ppms every res change to get starting ppm of nute mix. Then i would measure ppms every time i did ph checks. If i noticed the ppms going up while the water went down quickly then i was feeding too much and could adjust. If the ppms went down but the water didnt change much then i may need to feed more. I remember seeing a chart explaining the relationship between pH, ppm and water level but i dont remember where i saw it.
Third, check for algae or contaminates that could be causing ph to spike. If it is a new system did you clean the system before using? Not always necessary but a lot of times freshly manufactured and packaged items have thin layers of chemicals that can contaminate your system. As a rule of thumb, i cleaned all my new hydro materials with rubbing alcohol and rinsed and dried before using just to be safe. Could just be me being paranoid but hydro systems have to be clean and contaminate free or else whatever is in your res or water gets absorbed by the plant.
Lastly, do you have overactive air stones or lots of moving water? In my experience water agitation will raise pH levels if you have too many air stones or too much water agitation.
