Sent by GretchenEditor: I have also experienced this phenomenon and don't have a good solution. However, if you buy raw kombucha and leave it at room temperature for a day or so, the kombucha will start to ferment again and should create more carbonation in the bottle. Just don't leave the bottles unrefrigerated for too long or they may eventually shatter!
Readers, any other ideas for guaranteeing fizzy kombucha?
Related: How to Make Kombucha Tea at Home
(Image: Emma Christensen)