Your kombucha is not as carbonated as the store brands because  carbonation was added.There are two methods of carbonating beverages, natural carbonation and force carbonation. Most if not all the kombucha you purchase in stores has added carbonation. This method of carbonating beverages is know as force carbonation. Carbon dioxide is manually forced into the beverage with pressure until the desired results are reached. Force carbonation allows the brewer to maintain standards and provide a consistent kombucha. This method can take 1 - 48 hours depending on the equipment used. Natural Carbonation is what most home brewers use. Natural carbonation is also known as bottle conditioning and is a method where you add a little sugar or fruit juice to each bottle and let it sit in a warm environment.The warm environment allows carbon dioxide to be produced during the fermentation.  The co2 it has no place to go but to infuse into the drink, causing it to become  carbonated. To make your kombucha as carbonated as a force carbonating it can take 15-30 days of bottle conditioning.