All you are experiencing is the usual problem - you can't measure the amount of CO2 in tank water by checking the tank water pH and KH. That method will almost always give you a much higher concentration than you actually have. And, that is why the drop checker method was devised.
Your tank water has a KH of 4 and a pH of about 6.7, which would tell you that you have about 25 ppm of CO2. But, you actually have a lot less than that. So, the drop checker remains blue. (I'm assuming you are using distilled water with the KH adjusted to 4 degrees of KH with baking soda, and API pH solution in the drop checker).
The solution is to ignore the tank water parameters, and just raise the CO2 bubble rate until the drop checker changes to green. Until you do that, then set the pH controller to control to whatever pH the tank water has when the drop checker is green, you can't use the pH controller for anything.
And, don't forget, the drop checker response time is very long. It typically takes a couple of hours after any change in CO2 in the tank for the drop checker to indicate correctly. So, don't make big changes in bubble rate without waiting at least 2 hours to see what the effect of the last change was.
