We've found that most users reporting this have had the fanpage in question set to private.
You can check if the page is public or private by logging out of all Facebook accounts and navigating to the Fanpage URL.
If you are asked to login the Fanpage is private.
To fix this, go to your fanpage click on edit page>settings>age restriction to 13+ this then allows non logged in users to view the site (and Zapable is now able to find it).
Another common problem is that your Fanpage isn't actually a fanpage but is a personal page.
One more cause might be that the Fanpage is new. If it's <2 weeks old or hasn't much content in it, it may not appear in the FB API that Zapable connects to.