Facebook ad previews come directly from Facebook's servers. If your preview is not being displayed, the most common reason is that you have an AdBlocker extension enabled in your browser.
These kinds of extensions block ads from websites, but will also block ad previews when you're trying to create you own, so please disable them for the domain app.adespresso.com. For a tutorial on how to create an exception in AdBlock Plus check this page.
If the problem persists please wait a few minutes (sometimes the issue could be a temporary Facebook glitch) and if still not working, open a ticket. We'll be happy to look into it.