I just bought a Nizo s560 and have the same issue with the aperture blades / iris. When you place the switch to the BLACK side to test the motor battery that works fine. When you push in the black button at the exposure control in this mode, the iris blades also work and appear to react to different levels of light. I confirmed this by opening the side panel (very easy) and you can watch the blades in action.
So they're not stuck!
But the Nizo manual I found online says to test the light meter you put it on the RED mode which runs the camera and then press the exposure button (little black button on exposure dial). When I do this I get zero activity.
Then I inserted a cartridge with the side panel open and shined a light through the lens so I could clearly see the light going through the shutter and aperture blades. The cartridge insertion activates levers inside the camera so sometimes things don't "come alive" until those levers are activated.
Even after doing so, the aperture blades had zero movement when running film. They were stuck wide open. This caused all my film to get really overexposed.
My diagnosis at this point is that something is wrong with the electrical connectivity from the two small batteries for the light meter through to the blades. Why can't they just run the light meter off the motor batteries, would have been so much easier!
