A couple thoughts:
- Placement of the dc is key. You want it to be in the area that has the highest probability of low CO2, which is usually a region of poor circulation. This way you can be sure that you will work to optimize your circulation patterns. Did you pull the pH test water from the same area as you have the dc? Assuming, for the moment, that your dc solution is correct, that means that you have low CO2 levels in that area, which would be reflected in your x-point pH drop testing.
- DC solutions only last about 3 weeks PLUS it is critical to know what the dKH of the solution is. I make my own for this reason. If you are confident that your source of solution is correct, then I'd replace the solution in your dc. If circulation is uniform, then the the color of the dc is going to approximate the drop in pH from fully-gassed. Try moving the dc to different parts of your tank. It will take several hours before they equilibrate.
- BPS are only a relative guide, not an absolute measurement for CO2 ppm levels. However many BPS it takes to hit your CO2 target is what it is. Keep pushing your CO2 until you see your fish struggling, then back off and repeat daily until you fine the max point. Then you can see how your CO2 tests compare.
- The bigger the tank, the harder it is to get CO2 at the higher levels, with consistency, but you should be able to do it with the right reactor setup.