This is a known issue with Zed roadsters, usually associated with low ambient temperatures. The OEM canvas material is quite stiff at lower temperatures, so the "last" section of canvas doesn't fold properly and the roof won't close.
The Nissan handbook suggests that the roof should not be operated in temperatures of less than 5 deg C.
Usually the roof will close after a few attempts, as the canvas eventually flexes and folds away. The best option is to try to park the Zed in the sun,( I know that's difficult, especially this time of year!) but it is surprising how quickly the canvas warms up. Once the rear section feels slightly warm to the touch and flexes, then the roof will then operate.
It may, of course, be the fifth bow motor, as Zmanalex says, but, fingers crossed, it may just be the low temperature.
Once you are back in the UK, it would be worth while lubricating all the roof joints. I use Bilt Hamber Ferrosol on my roadster linkages and joints and can recommend it.
Finally, never, ever, try to force the roof down by hand as undue force can damage the motors.
Lets hope for some slightly warmer weather, so we can both get the roof down and enjoy our Zeds!
Hoe this helps.