5 dKH distilled or deionized water in the drop checker would be green at about 40 ppm of CO2. I don't think DIY CO2 will ever get that much into the water, so I think there is something wrong with how you are using the drop checker. It could be that the KH of the water is much lower than you think. It could be that you aren't using distilled or deionized water. But, BBA usually means either low concentration of CO2 or water circulation in the tank that is not adequate for keeping CO2 enriched water available to all corners of the tank.