C7 Spare Key not recognised

Hi guys.

I have a 2012 3.0 TDI S Line Q Avant. Non advanced key, just standard keyless start.
Every time I try to start the car using the spare key it says it is not recognised and needs to be held against the key logo on the dash. When I do this the car will start but I have to do this every time, it doesn't seem to remember it. Now I can't open the doors remotely with it.

The red light on the key flashes when it is pairing to the car but afterwards if I press any of the buttons the light stays off.

Am I doing something wrong? Driving me mad!

Thanks

Thought that might be the case so ordered one. Would the key just not work at all if that was the case though?

Brilliant. Love it when it is something simple. Will keep you posted.
Thanks