One of the reasons your kombucha may not be bubbly is because it’s fermented for too long. The good yeasts in kombucha eat the sugar and then release carbon dioxide. This is what creates carbonation. If you let it ferment too long the kombucha go flat. If you don’t let it get vinegary, it will stay bubbly. Second-fermenting is another way to achieve carbonation. The process requires the extra sugars and the juice in the capped off bottle, so the yeast can transform the sugars into natural carbonation.