Try different words that mean the same thing. Some search engines allow complex searches using combinations of many terms. Each search engine has software which constantly roams the Web to gather sites for its unique index. Each search engine gathers sites at its own rate in its own way. Search engine indexes are updated at different schedules.
New sites show up in search engine databases at different times.
Your search term may be relevant in one search engine database and not so relevant in another. Subject directories are much smaller databases of Web sites and each entry has been examined and sometimes evaluated by a human being and assigned to a subject category. There are thousands of specialized databases available containing information not indexed by search engines or subject directories. For example, The San Francisco Chronicle provides a searchable database of the full-text of staff written articles since 1995. This information is accessible only through the search engine at the Chronicle Web site.