First and foremost, it's comforting to know that the amount of carbonation in Kombucha Tea has practically no impact on the ferment's efficacy. I say virtually because the effect is minor, and there are reasons both for and against carbonation's effects. I should also point out that the presence of fizz in no way means that the fermentation was effective. If you have a thick SCOBY but no fizz, that's a good thing because you have only enough yeast to keep the bacteria satisfied but not enough to overwhelm them.
So, what can you reasonably expect from a "natural" Kombucha Tea fermentation? It is customary for Kombucha Tea to reward you with bubbles after 3 or 4 cycles of fermentation. It's possible on the first cycle, but it's entirely dependent on how the SCOBY you obtained was created, how the SCOBY was handled before you got it, and how you handled the SCOBY.
A "balanced" ferment - can create a lot of bubbles, which you'll see when you pour the kombucha into a jar and the kombucha foams up, close to what washes up on the beach on a windy day. It can taste like beer in extreme cases, but this is rare and may mean an overabundance of yeast. When sipped directly from fresh ferment, the bubbles would usually be scarcely visible by the palate, but it will have a consistency that is very different from flat Kombucha Tea and much more appealing to the palate.
Flat Kombucha Tea is common among new users because the yeast population lacks variety or hasn't reached the critical number for producing good fizz. They will finally appear after many brews; some are airborne, while others may be hesitant to replicate due to dormancy. In the meantime, there are a few things you can do to encourage them.
Under two conditions, yeast usually switches from CO2 to alcohol production. The first is a deficit in surface and dissolved oxygen, while the second is an excess of glucose. As a result, before setting up a ferment, wire whipping or other types of aeration, as well as using a little less sugar, can help increase carbonation in the early stages of fermentation.
Adding a little more sugar after the yeast has completely populated will get them moving. This can be accomplished halfway through the fermentation process or right before bottling. If you do it during bottling, be careful because it just takes a small amount to make a grenade! A quarter teaspoon per quart is my suggestion. Some people add a few raisins or ginger slices to a 250ml to 300ml bottle, which both work well.
If your kombucha brew is still sweet when you remove it, leave your Kombucha brew to ferment for longer until it is more tart. After a few more days, you might see a few tiny bubbles forming and rising to the surface. Using a torch to take a closer look at your kombucha brew you're able to see one or two bubbles, this is when you are ready to bottle.
Keep your Kombucha bottled at room temperature for 5 to 7 days before refrigerating for amazing carbonation. For the best performance, refer to the link on flavouring your Kombucha which explains how best to carbonate when flavouring. Keep in mind that if you store Kombucha Tea not in an airtight bottle, it will eventually go flat, just like any other beer or fizzy drinks.
When you make your next batch of Kombucha use the starter liquid from the bottom of the last fermented kombucha batch to make your next kombucha batch. There will be more yeast in your Kombucha and the more naturally produced yeast results in better carbonation. Use at least 20% starter until the fizz starts to improve, and then reduce to 10%. Allow 3 or more days for your starter to sit open at room temperature (cover it with a cloth or paper towel.) This ensures a high level of activity and gives airborne yeast a chance to settle
When bottling ensure you leave as little air in the bottle as possible to increase the carbonation of the Kombucha, the less air present in the bottle the more of the CO2 will be dissolved into the Kombucha rather than the air.
Try some different types of tea if you've just been drinking black tea. Green is better for carbonation in general, but you might want to blend it with black for a better taste. For better performance, I suggest a 3 to 2 ratio of green to black.
If possible, raise the temperature to about 27 degrees before the yeast takes hold and the fizz starts to appear, I suggest using a heat mat. Then reduce the temperature to 22-24 degrees to ensure that the ferment's equilibrium isn't skewed too far against the yeast. This can be a far more serious concern that will take time to address. Some researchers believe that 23 degrees is the ideal temperature for SCOBY equilibrium, and my own experience seems to back this up. I like the increase in taste that lower temperatures offer, but lower temperatures also mean less carbonation.
Remember the best bottles for carbonating kombucha are the amber squealer and amber swing top bottles not only because they are airtight and ensure maximum carbonation; they also protect all the healthy bacteria in your kombucha from UV light which can harm/damage your kombucha and all the health benefits.