> update all optional and important updates from windows update, this normally helps
>If you didn't find KB3035583 in automatic updates, try to download it using a browser it gives you system readiness information and starts alerting your for windows 10 invite.
> As a last resort, you might try the media creation tool for windows 10
This tool serves two purposes: (a) upgrade your pc in place, like the invitation does (I suppose) (b) creates an iso or usb disk to update windows 10 on any supported device (beware, you will need your windows key to proceed with this tool and it might help you do a clean install - without leaving any copy of your existing windows 7 or 8)