In order to get the tray icon to appear:
First: you must have a legitimate license for those machines (assume you do)
Second: You cannot have "Enterprise" editions installed on said machines. You have to have anywhere from Standard/Basic/Pro/Ultimate editions.
Third: have the current Service Pack/updates...
- On your Windows 8 computers: Ensure you have the update to 8.1
- On your Windows 7 computers: Ensure you have Service Pack 1 installed
(both of the above are available through Windows Update).
Also, there are a number of Optional Updates that include patches to various optional components to your OS. Ensure these are installed as well... not JUST the required updates. (Things like Skype and Silverlight are not essential, but other updates and patches to things like .Net etc are)
After you complete these updates to your current OS, in about 24-48 hours the tray icon promoting Windows 10 should appear.
I've done 6 machines now, a few were fresh installs, and a couple were currently in use. After the updates, I eventually got the icon on all of them.
Cheers.