The amount if fizz or bubbles is not generally indicative of a successful brew or not. It also has minor impact (if at all) on the overall benefit you get from drinking Kombucha. Some of the best brews are flat.
Typically when you start out, it’s more common to not have enough fizz. That’s because the yeast is what creates the (CO2) fizz and the yeast might not have enough variety and strength. After successive batches, it will pick up more airborne yeasts and some of the yeast that has become dormant will wake up
Too much sugar, weak tea or not enough tea, and cold temperatures can contribute to lack of fizz.
Usually yeast will increase as you do several batches, and many times you start running into problem with too much yeast, fizz and bubbles. But if you want to try tweaking for more carbonation, you can try less sugar, more tea or different tea and increase the temperature if possible.
Perhaps the best remedy to a flat kombucha drink is to add a tiny bit of sugar right before bottling. Be careful not to do too much as it can really get it going.