It is not unusual for many solenoid valves to require some minimum 'upstream pressure' to allow the value to actually open. The solenoid mechanism just allows a gate to open (unlatches) but relies on upstream pressure to actually move the valve open. There are solenoid values that will open with 0 upstream pressure. One needs to review valve datasheets carefully for this kind of details as well as if rated or gas/liquid and material compatibility issues.