Carbonation is created when yeast converts sugar into Co2 and alcohol - if you want to increase the C02 (fizz), there are several things you can do to increase the carbonation in both the first and second fermentation.
First, let the first fermentation brew for longer. Allowing your initial brew to go for at least 14 days will strengthen all of the activity in your current brew and create stronger starter tea moving forward (making your future batches better). At the end of your first fermentation, there should be at least a small amount of natural carbonation that has built up.
If you'd rather only do the initial brew for 9 -14 days but still want some good carbonation... then bottling for a second fermentation is for you!
Try these second fermentation steps to get your kombucha bubbly:
1) Fill your kombucha bottles closer to the top. Leaving no more than a half an inch of space. By reducing the amount of oxygen present in the bottle, more Co2 is dissolved into the Kombucha.
2) Increase the sugar content in your second fermentation. Either add fruit or fruit juice in to each of your bottles (if you've already tried this and it didn't make it more carbonated, try adding 1 tsp sugar to each bottle).
3) Allow the second fermentation to go longer, try 4-6 days. Do note, you will need to start burping the bottles once a day after about 3 days of fermenting to keep them from exploding (it's rare, but it happens). Simply open each bottle slowly to release the Co2 and recap.
4) Your Second Fermentation should be at room temperature. As soon as you place a bottle in the refrigerator, fermentation, and therefore, Co2 creation, will cease. If you like it bubbly, don't place in the refrigerator until its reached the carbonated level you enjoy.
5) If you are using old Kombucha bottles from the store, try placing a small square of parchment paper underneath the cap to get a tighter seal. Swing top bottles are the best bottle type for building holding carbonation.
6) Lastly, realize that your brew may never reach the highly carbonated levels of grocery store Kombucha or soda because they add artificial Co2 during bottling. Unfair expectations!