Hi, WormSlayer!
What I'm seeing is that, if you have shared your compendium in a campaign that isn't the one you're looking at, you can hover over it and it turns red with an X to remove sharing from that game. That's so it's easy to free up a sharing slot if needed.
Hovering over the "compendium sharing slot available" gives the red strike-circle. That's because those empty slots don't have any actions you can perform on them.
To add sharing to a game, you go to that game's settings, click "Yes" under "Share my compendium with players" and then scroll down to the "Save Compendium Settings" button, which is below the compendium expansion selector that shows all the expansions:
If you instead scroll all the way to the bottom of the page and click "Save Changes," the compendium settings won't change. This is a known issue for now.
Edit to add: If that doesn't solve your issue, or if something else is going on, please comment back or hop over to the Help Center to open a ticket.