I have to check the details in my American Farmstead Cheese book. But I have had similar issues in the PH from farm to cheese vat to renneting. As the guy from Danlac said as long as its moving the right direction you should be OK.
From what I have read, the milk from the cow should be at about 6.7 to 6.8, when I check at milk pail at the farm this us usually the case (I have had some exceptions and talked to my farmer and he has since admonished his contract workers to rinse the euipment better after washing it ) seems that detergent remnants can raise the PH and I got readings of PH 7.03 one day at the farm (this was an exceptiona and since the PH in the pail has always been on target). I find that as I drive home from the farm, the adgetation, cooling and aging of the milk will raise the PH from around 6.7 to almost 7.0 most of the time (usually about 1-2 hours before I get it home and in the cheese vat). From what I have read the PH will normally raise a bit if the milk is roughly treated and when they do the initial cooling into the storage vat at the farm. I have measured the PH in the farmers cooling vat and it is usually around 6.85. Ialways take my milk from the milking pail as I want to make sure that Iam always using milk from a single (and just finished) milking. I trust straight from cow more than anything. My milk never really cools, perhaps just a bit from the aircon in the car.
This always concerned me because I have read that at a PH of over 7 the cultures would not become active. I sometimes add some calcium cloride as I read somewhere this can act to bring down the PH in slightly basic milk.
I have found if I am on the right side of the PH of 7.0 I can go ahead and things get going. I do watch the PH very carifully; if the curds get too acidic durring cooking or stirring (e.g., less than PH of 4.
it can become very difficult or impossible to get the cuds to knit in the press. If things are going south too fast on the PH, I quickly will get the cheese pressed even if it is earlier than anticipated.
I really rely on the PH and the chemistry now much more than timings in the recipes I see (except for timings regarding temp holdings and temp changes required). Although, I had to cheese last weekend without my PH meter - as I had dropped it Brine and it got fryed; my daughter arrived from the US last niht with a new waterproof PH meter with her. So this weekend it will be back to watching, noting and acting on the acidification process.
I will pull the details if I can find them from the book when I get home later. I have never worked with store bought milk so I do not know what the differences you may find with your milk.