A KH of 20ppm is still fine, 30-40ppm is fine also.
I generally will not add KH unless it's less than 20ppm.
I've done tanks with no KH also, but had to add the KH to get a ball park range of CO2, then removed all the KH later.
You may just need to add more CO2 is all and the accuracy of the pH test kit might be part of the issue. A pH pen or pH monitor might be a wise item to get. also, make sure to turn off any electrical devices before taking a reading and calibrate the probe often.
Do not just measure once either, measure several times thoughout one day to see how the CO2 vary over a typical day(measure just when the lights and CO2 are on).
Afterwards, this will give a fairly good idea what the CO2 is doing and you can predict what ranges the CO2 is. then you will not need to test much again so much unless there is a problem with algae, or plant growth slows down dramatically etc. Then the pH pen/monitors make a quick spot check from then on fairly easy and quick.
Do not think of a any measurement, CO2 in particular of all the measurement test we do, as a stable level throughout the day. It varies, so just seeing one point in time does not tell you that much. It's more work to test every 1-2 hours, but it does give you a much better idea of the patterns with CO2.
It's not something I'd suggest you do as a routine though.
Many folks assume their CO2/opH is the saem if they take one single measurement and post that, some folks post a range of pH's for the day(better).
Now that is just day variation, add week variation or monthly variations and folks can have a very wide difference.
If the CO2 is stable over the month etc, then you have pretty nice growth and little/no algae.
Regards,
Tom Barr