I have been brewing Kombucha for about two years. I tried growing a few Scobys in small jars to give to friends, but I noticed that the liquid in the jars, which should have been mature Kombucha an taste very sour was instead neither sour nor sweet. The Scoby on top has grown but I am worried about the potential lack of acetic acid and how this affects the culture.
During the fermentation of Kombucha, yeast (a S. Cerevisae strain) turns sugar into alcohol, and a bacteria (an Acetobacter strain) oxidizes it into acetic acid (vinegar). There's more going on that just that, but these are the parts of the fermentation relevant to your question.
If you have no acetic (vinegar) flavor, the oxidation of alcohol into acetic acid has not (sufficiently) occurred. This can be caused either by a shortage of oxygen in the liquid (if the SCOBY covers it completely and has grown very thick that might impede oxygenation) or by a microbial imbalance in the SCOBY (the acetobacter population is very low). The fact that you have no sweetness means the yeast has fermented the sugar in alcohol, but that's an anerobic process so it does leave open the possibility of oxygen deficiency.