I would increase it.
To me, the 'no pH change' suggests the plants are using it as fast as it enters the tank.
Increase it slowly, watch the livestock, and monitor the rate to see that it is consistent.
If everything is OK, but the drop checker shows that the pH is still a bit high (indicating not a bit increase in the CO2) then add a bit more.
Make the changes at least 24 hours apart to be sure you are really seeing the results.
To me, the 'no pH change' suggests the plants are using it as fast as it enters the tank.
Increase it slowly, watch the livestock, and monitor the rate to see that it is consistent.
If everything is OK, but the drop checker shows that the pH is still a bit high (indicating not a bit increase in the CO2) then add a bit more.
Make the changes at least 24 hours apart to be sure you are really seeing the results.