Hmmm, Windows 7 Pro? Check. I made sure I ran all the updates, I was checking my hardware compatibility, checked with work that the update wasn't blocked centrally and even went through all the suggestions in these two help articles: https://answers.microsoft.com/en-us/windows/forum/windows_10-win_upgrade/i-want-to-reserve-my-free-copy-of-windows-10-but-i/848b5cce-958b-49ae-a132-a999a883265b and http://www.makeuseof.com/tag/windows-10-upgrade-notification-enable/. All to no avail.
Then I realised that I hadn't had the notification on my home computers either. So it pointed to it being definitely something I'd done.
Now, what was the same across all my computers...? It couldn't be could it....? Yes, that's it!
I had Internet Explorer disabled. So, turned it on, and a few updates and reboots later, there it is. 'Reserve your free upgrade'.
So there you have it. If you don't have IE enabled, you won't get the upgrade notification. Hope this helps.