Tried this on the FAQ thread but perhaps it's better here...
Can somebody explain this new url format? I've noticed people posting links like this
The Unreality of Time - Wikipedia
en.m.wikipedia.org
But if I use the same tag structure with unfurl="true" in the URL tag, then it doesn't work:
Begging the question - Wikipedia
en.wikipedia.org
Unfurling is the term for gaining some information about a URL in order to display a preview as you can see in your post. I have found that this doesn't work for all URLs; sometimes there's no preview even when unfurling is enabled. I suspect that, as @sun rise said, this could be related to the link itself and the kinds of data the software is trying to retrieve (and probably not finding) from it.