This might be too late you for you but here goes:
pH is temperature dependent with the mechanics of the meters used. If your meter is not adjusting for temp then it is most likely not accurate when the milk is cold, this is why you are getting falsely high readings. I am also assuming you are calibrating regularly. Also be careful with cleaning your probe, pH works by hydrogen concentration and wiping with a cloth can cause static build up on the probe, skewing results.
Now for your knitting:
Most likely causes of poor knit are excessive cooking, loss of too much heat during pressing and insufficient pressure during pressing. Traditionally parmesan is allowed to "settle" in the hot whey which creates the tight texture of the curd and it is lifted enmass into the mold. The curd must be very hot for this recipe to knit well. Don't be afraid to flip, flip, flip in the press. As the pH on the surface drops it will toughen and your finsh will get smoother and finer.
Also, for pH drop, you are looking for 0.1 minimum, with 0.2 preferable, between renneting and hooping. Generally you want to rennet at 6.5 (about). This depends on many factors but is a good ball park for mesohilllic recipes. For this recipe you want to just get the thermo primed, so say a 30 minute wait, then you can add rennet and cut right at floculation.
Good luck in the future.