WiFi Extender won't accept same password as router

We have successfully connected the WiFi extender to the Plusnet Technicolour Router TG582n several times but every time we put in the password for the router to connect to the extender from a device it says we have the wrong password. We have checked this password several times and it is the correct password for the router, so it doesn't make any sense. 

We have logged into the Router using IP address but can't find the extender's IP in the list of connected devices for the extender to try and reset the password that way. I have reformatted both the extender and rebooted the router to try and get it to work but no luck. 

I'm at a loss. Has anyone had the same issue/know how to resolve this?