How much turbulence do you have in your tank? How quickly does your pH tend to drop? Depending on your plant growth and the gas exchange at the top of the tank, it wouldn't surprise me to see that it could take a day or more for CO2 levels to drop appreciably.
Try this. Take your drop checker out of the tank and let it sit outside the water for a while (maybe 20-30 minutes). This should cause the solution to go back to blue. Assuming it goes back to blue, then attach it back into the tank. If it returns to yellow, I'd suggest that your CO2 levels dissolved within the water are still in the 30+ range. Give it time. I would think it should change colors when the water conditions have sufficient time to change.
As I'm fairly new to the drop checker business, I'd love to hear those with differing experiences,