This Gauge shown the air pressure in the liquid cylinder.
Our engineer said, 0.4 is too high, try to adjust it lower. Because pressure of mixing machine is low, pressure of filling machine is high, it is not easy for filling machine to fill liquid.

You said, there is not enough carbonated in the Can, our engineer offer the solution:
1, Keep the temperature of water chiller is 0-4℃. Highest temperature can not beyond 4℃
2, Adjust the pressure of mixing machine is higher
3, Keep there is enough water for CO2 steel cylinder. Because the temperature is lower, if the CO2 is frozen, CO2 can not flow into mixing machine.

The pipes for mixing machine
https://www.youtube.com/watch?v=xHnG5vdRf0Q