Something is inconsistent with your water parameters. If your pH is as low as 5.8 you almost certainly have a high CO2 concentration in the water, so a properly set up drop checker should be light green to yellow. If your KH test kit is giving you the wrong reading and your KH is really closer to 1 than 4, then this could be consistent. Or, if your pH test is from using a pH probe that isn't well calibrated, the pH could be closer to 7 than 5.8, and that would be consistent. Since your drop checker remains blue, after a couple of hours of CO2 injection, it is very unlikely that you have more than about 10 ppm of CO2 in the water.