I had the exact same issue with my H3DII-39 before I knew more about using it on a tech camera. You need to wake up the back before you take the shot, even though Hasselblad says it's not neccessary. As Enda says, you need a Kapture Group wake-up cable. Even if it's just a fraction of a second before the shutter fires, the back needs to be sent a signal first.
Another, slower way of shooting is: Buy a cheap cable release on Amazon that you'll connect directly to the back (search for a Neewer eza-c1). In the back's menu, set an exposure time longer than your shutter speed. When you want to shoot, trigger the back first, then take your shot while the back is exposing. The shot must start and end inside the duration of the back's exposure. The back needs to see dark at the start and the end of capture.