This can have several causes. Did you use too little sugar? Are there enough yeasts? If your glass is aerated only with the bonnet, the carbonic acid can always escape.
The second fermentation in closed bottles is great for this. Fill your kombucha into bottles and close them. After 24 hours, there should already be a good amount of fizz on it.
Your kombucha is still not carbonated? Try feeding it with some sugar after bottling before you seal it. Often it also helps to add some (dried) fruit.