Try these second fermentation steps to get your water kefir more carbonated:
Increase the sugar content in your second fermentation, adding high sugar fruits/juices like apple or orange!
Allow the second fermentation to go longer. Do note, you will want to ‘burp’ the bottle every 12 hours to keep it from exploding. Simply open the bottle slowly to release the CO2 and recap.
Ensure you are keeping your ferments consistently warm enough, between 20 – 30 C (70 – 80 F. Don’t place the second ferment in the refrigerator until it has reached the carbonation level you enjoy.
Lastly, realize that your brew may never reach the highly carbonated levels of store-bought soda because they add artificial CO2 during bottling.