Norton Download on a new laptop using existing key

Hi

Currently having an issue with my existing subscription, which is only due to expire early next year. It can be made active for 5 devices, but we have only activated it on two. I have bought a new laptop and i have tried activated it with the existing key, but it says its already active, and need to use another. The history of my subscriptions aren't showing on the website since 2013 either. 

Does anyone know why it wont let me use the existing product key? I am using a windows 10 machine, not that this should make a difference.

Thanks in advance.

Matt