It sounds like either the nutes are not getting ballanced when you PH the rez before putting it into service, or the plants and coco are eating up the chemicals that are more acidic and leaving the more alkaline chems behind to raise the PH. That is not at all unusual with coco and MJ. I just filled up a 10gal rez day before yesterday and had the PH set at 6.1(knowing that they are in bloom and will use the more alkaline nutes and cause the PH to drop) when I checked it today, they had used 1/4 of the water and the PH was 4.8. I had to jack it back up to 6.1 again as I know they will continue to pull it down.
How big is the rez that supplies your plants? When you mix up a solution for them, do you let it rest and/or aerate for a while before putting it into use? I understand that some chem nute brands put PH buffers in with their stuff and you have to let it sit for 24hrs before even checking and adjusting PH, then you have to let it sit again for at least 6hrs after adjusting PH before checking it again.