Bubble rate is not really important to a planted tank...what you really need to know is your pH/KH values to find CO2 level. Dial the gas to reach a certain pH for a particular KH vice versa.
As your problem, it might be heavy back pressure thus reducing the amount of CO2 gas being pushed through? Are you using a diffuser? For me, my bubble rate usually starts slightly lower but reaches my preset rate after 1/2hr or so.
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