12 V Solenoid Valve Not Opening
This is a typical industrial datasheet, in that it's light on the details.

The mechanism isn't obvious from the picture (can't tell if it's a pilot valve), but it probably does need a bit of pressure to open. You haven't told us your operating pressure, but somewhere between 0 and 5 psi may be necessary.

Next possibility is that your valve is broken, but I've never seen a solenoid fail in that manner so I think that's unlikely.

Another possibility is that you've connected the valve in reverse. Depending on the valve, the direction of flow may be important.