For what its worth, my ph low point on mine is 6.2 because I needed it to melt faster to keep up. I wouldn't do that yet in your case, but try the above first.
Reason for that is you say you're set at 6.4 but reactor ph stays at 6.5
That tells me its not enough time do to flow rate. Or if your using a carbon doser valve turn it down a tiny bit to allow more co2 in at a faster rate to get reactor ph down