Any of all those Full Library sections are saving filter and order settings!

Was it purchased through All Access? If so, check under either Settings then Account or Games and Apps then Full Library for a section for "Include with this Xbox" and it should list the code there.

Check in account settings or under full library in Games and Apps for a section for "included with this console" and the code should be there.

Yes there are more than one account on the console but I own all of the games as the other accounts are family accounts and all games get installed under my account. The only Games I have hidden are 5 Demo games which I have noted under the Hidden Games
element.

But none of that would explain why there are 20 more games in the My Games and Apps section and yet there are 20 less in the Full Library section. Surely the Full Library should have more as is called the FULL Library, not Partly Full Library, or Nearly
Empty library.

If the game isn't hidden, as the above poster mentioned, it's possible that perhaps you've ticked some of the filters within My Games and Apps which is why some of your games aren't showing up. When you're in the My Games and Apps section, and in your Full
Library, scroll over to "Filter" and double check that you're looking at "All Console Types".

It's most likely to be a game you've hidden by accident (or somebody else hid by accident) though.

If the games are already on the console and you just need the game saves, start the game. The game will sync with the save on the cloud. Again, the cloud is the only way to transfer game saves (not the games themselves).

If you need the game itself, then you'd check under Full Library in Games and Apps and filter by Ready to Install and any games you own that are compatible will show there and you can install them from there.