A little more info which may help?
As mentioned above, without an else statement the query returns the single most recent post. If the most recent post happens to be the one post assigned the ‘preview’ custom field, with a url value, then the image does show up where it is supposed to for the featured post.
So it seems the custom field and its value are present/associated with the post, so how is it that the query isn’t identifying this and pulling in the custom field post, rather than the one most recent post?
I’m confused.
Any help greatly appreciated.