During the warmer months of the year, our Water Kefir Soda did wonderfully well, and all I was feeding it was Sucanat and then the second ferment was with grape juice. Marvelous stuff. Then came winter. The change was very gradual, but as the temperatures fell, the carbonation in the soda eventually reduced to ZERO. I remained puzzled for a few weeks and then took another direction. I ran out of grape juice and started using apple juice and I also added a dried apricot (nearly cut in half to increase exposure) to each batch of the brewing water kefir. The change has been nothing short of amazing! Now there is plenty of carbonation. I am far from an expert on this, but want to share what is working for me currently:
In a quart jar, place 1/2 cup water kefir grains, 1/4 cup sucanat, the dried apricot and then fill it with non-chlorinated water just to where the threads start at the top.
Cover loosely and let sit at room temperature for 24 hours.
Strain out the liquid, discard the apricot (believe me, the taste is gone).
Pour the liquid into flip-top bottles, add half again as much apple juice. Cap, and let it sit at room temperature for another 24 hours and then refrigerate.
Start your next batch as soon as you drain the first.
Be sure to carefully rinse your grains between batches.
The grains grow amazingly, so I keep the extras in the refrigerator in a covered quart jar with the water and 1/2 cup sucanat. I change that out, rinsing them, once a week.
I suspect that the variations in climate in your kitchen will require that you find what works for you.