Let's troubleshoot the kit first:
You're using 5mL of tank water, then adding 4 drops of CO2-1, correct?
Then, as you add CO2-2, do you gently swirl between each drop?
If you are gently swirling between each drop, how many drops of CO2 are you adding until the sample (after a gentle swirl) becomes very light pink?
Troubleshooting the drop checker:
First, remember that DCs take 1-2 hours to change. They are always, always 'delayed' by that 1-2 hour transition.
What kind of drop checker are you using?
The regular kind:
View attachment 13902
Or this over-the-tank kind?:
View attachment 13903
Answer these questions and I'll help you out!