[MrForbes]

If you are activating it by manually switching the power that is connected to the solenoid wires, what exactly are you doing?

I suggest you momentarily connect the red and back wires from one end of the solenoid valve to the power source, then disconnect them. Next, momentarily connect the wires from the other end of the solenoid to the power source, then disconnect them. If you have the plumbing right, and you have sufficient pressure in the air supply (more than 30 psi) the solenoid valve should shift and the air will move the cylinder.

edit: and yeah, you need to use 24vdc to get this year's solenoid valve to work!