Is this milk or water kefir?
If it is milk, then I think the sweetness is due to the added sugar. Usually the kefir is tart due to the sugars in the milk being used up, and lactic acid being produced, but since you're adding more sugar to the mixture, it might be masking the tartness of the brew.
I'm not extremely knowledgable about water kefir, but it sounds normal to me that the resultant brew is sweet rather than tart. You might just have a high kefir to liquid ratio, which is alright, but it's usually better to clean and change the milk/water of the brew every day or so, rather than make large batches. After long periods of being exposed to acidic environments the grains of the kefir tend to build up walls and go yellow, meaning they're slowly dying off, so watch out for this!
But it's usually a very good sign if the grains are growing like crazy.