Maybe I just missed seeing it, but what is the size of your tank? If it is a 10 gallon tank, the amount of CO2 you are using could be too much, but if it is a 65 gallon tank the amount is probably too little. Unlike Tom I think a drop checker can give you a good approximation of how much CO2 is in the water. But, it only works if the water in the drop checker is distilled water with a little sodium bicarbonate in it to give you the KH you want in the drop checker. If you try to use tank water the color of the drop checker will mean nothing.
My tank is a 55 gallon.
I did a quick Google search on adding 4dkh solutions to drop checkers and I realized how little I know about drop checkers even though I have been using them for years.
The drop checker I have is similar to this except it is Chinese:
The solution it came with told me to add 5 drops. I do not know if it is a 4dkh solution.