Try different keywords and search terms using different databases and catalogs. Every database is different so some keywords and search terms work for one database but not for another.
Expand your search:
Include synonyms and plural/singular forms of keywords. Separate synonyms by OR. Separate the synonyms from the rest of the words by using parentheses.
Use truncation symbols (or wildcard symbols) to include variations of your search terms (e.g. scien$ will search for sciences, scientific, scientfically, etc.).
Narrow your search:
Combining different concepts/search terms with AND
Use the limit functions of the database. These are often located on the left side of the results page, or look in the database's Help menu to discover the limit functions it offers. Possibilities include limiting by date, language, type of article, etc.
Keep a record of which search terms worked and in which databases. This can keep you from repeating your steps.
Did you find an article you really like? Then, read the cited references (a.k.a. bibliography, end notes, footnotes) to find similar articles. This can bias your project by focusing on only one side of an issue so use caution with this method.
Ask for help. Ask a librarian for search tips. Also, use the help screens in the databases for instructions and tips.