Drop checkers dont change for a bit, though shoudlve within 6 hours. First off, use pH to estimate CO2, since its much more accurate than that drop checker. Also, make sure distribution of co2 is good. So improve flow. Finally, try to not lose too much co2. So this involves things like no air stone, not too much surface agitation but enough to have a ripple. You're looking for a pH drop of about 1.