There is a known error within the HDAS databases which can cause an "article not found" error when trying to link to the full-text of an article returned in search results. See example below:
This is occurring because there is a mis-match of data about the article / journal title between the list that HDAS relies on (the OCLC link resolver) and the provider/publisher data (eg Proquest or Medline Complete etc). We have noticed that this can occur when the journal includes online-only content as well as providing a print issue.
It is most likely that the article full-text is available within the provider / publisher content - you can try to search for the article title there.
Please let us know of any instances you come across, we can then ask the provider / publisher and OCLC to resolve the data mis-match.
Known NHS eJournal problems are recorded on the Journal Problems and Fixes web page.