Check the article for keywords. There might be synonyms for your keyword that researchers use. There might be additional keywords that will help you to narrow the scope of your search. Keywords are usually found above or below the abstract. Redo your search with these new or additional keywords.
Look through the article's references. You should be able to find citations to older, relevant articles there.
Search Google Scholar, Scopus, and Mardigian Search (the search box on many of the library's web pages) for the article you already have. Look for a link to "Cited by", which will give a list of articles that cited the one you already have.
Search for articles by the same author(s). The author has almost certainly written other articles on the topic.
Search the same journal for more. If a journal has published one article on a topic, it's a good bet that they have published others.