The response time for the CO2 checker can be rather slow.

Older tanks often will have fairly stable pH's, if so, then you might consider deep vacuuming the substrate in 1/4 sections each week to clean it out good and then replant.

The accuracy of the color in the drop checker is only so so, but even with a pH monitor/probe, I've seen this happen.

Not quite sure, but it does tend to bother me some and I will clean out the tank good if so and do a large water change.
That seems to solve the issue.

Also, when plants are growing well and taking things in via the roots, they add acids to the substrate, they do this to make nutrients there more mobile to remove them for uptake. This can have an effect as well. Hard to show that in situ though.


Regards,
Tom Barr