First, this is a great plugin! We use it on several WP sites and love it. For some reason though, some of the Facebook pages only display the custom tabs we've create to the app author, or in some cases the page administrators. It doesn't seem to matter what browser is being used either. I verified that I do not have sandbox mode enabled too.
You can see the tabs that are on the page, but no content shows up.
We self-host on our secured server, but it shouldn't work for any of the sites if that were the issue.
Here are two examples: