Neutro Bromo Blue solution can be used with all drop checkers and is the only solution you should be using in a drop checker. Does not require mixing or anything else.
Directions: Add a few mls to your drop checker, then pop the unit directly into your aquarium and wait to see the colour change. Ideal colour to aim for green. Blue means too little, yellow means too much. Adjust your CO2 output accordingly.
You may also want to consider the 3ml Pipette which is used to dispense the Bromo Blue liquid into the drop checker.
If your drop checker stays blue you simply need more CO2. Simple as that.