You could try leaving it for longer.
You could try changing your sugar to regular white sugar.
Or you can try a special mineral supplement added to the water.
Do you use filtered water?
Are the grains multiplying? If your grains are multiplying, they are feeding on the sugar and growing.
I use 1 tablespoon of sugar, about 3-5 tablespoons of grains now (I just let them multiply), and around 2 cups of water I think.
Mine doesn’t really taste sweet – only very slightly.