If you are using one of these systems : Cell xCellence, SP8-IN1 or LSM700, the main CO2 valve is now attached to 3 individual ones:
The main valve should always be open, and you should only work on the individual valve (blue) which is controlling the system you booked:
{F22946832, size=full}
NOTE:Please do not forget to close the individual CO2 valve when your experiment is finished, but LEAVE THE MAIN VALVE OPEN: someone else may be doing live imaging as well!