There are two potential answers:
- Your search may be configured in a way that is preventing it from being retrieved, please double check the search criteria (do you have too many required terms? Do the required terms exist in the article in question?).
- The source may not have a lot of authority. That is, there may be very few references to the source on the web and that will make it harder for us to find the content. In general, the more popular the mentions source, the easier it is for us to find the content.