Are you using the CaMg+? That should raise the pH of the solution a bit.
Really, though, what goes in doesn't matter so much. Check the pH of your run-off. In soil, you would add dolomite lime to your mix as it neutralizes acidity over long periods of time and provide Ca and Mg (soil microbes and the plant itself also adjust the pH in the rhizosphere). You can add dolomite to coco coir also, but the best brands of coir will already be in the 6.0-6.7 range.
The General Organics CaMg+ is "biologically catalyzed from oyster shell, dolomite lime & natural plant extracts" so essentially a liquified dolomite lime. Dolomite is calcium/magnesium carbonate whereas oyster shell is primarily calcium carbonate. Use it at recommended rates if you have not been already.
Another thing to consider is that each one-part nutrient (Bio-Thrive Grow\Bloom) contains humic acid and probably has other organic chelates as well. The Diamond Black (optional on the feeding chart) is also pure humic acid, has a pH itself of 10 and will make the solution very dark and thus difficult to get an accurate pH reading with the liquid drops. Humic\fulvic acids and other organic chelates help plant roots take up nutrients that would be otherwise difficult to assimilate.