Search engines are only as good as the software programs (robots/spiders) that run them. Search engine software is only as good as the programmers that write the code.
Search engines are created by various human beings, thus they each work a bit different from one another.
Similar commands can have different search effects on different search engines.
Each of the major search engines has indexed less than twenty percent of the Internet (as of December 1999). Collectively, the major search engines have indexed approximately sixty percent of the Internet.
Each search engine has indexed a slightly different portion of the Internet than other search engines, with some overlap occurring between search engines.
After using a search engine, if you don't find what you are looking for, that doesn't necessarily mean that it doesn't exist online. It may only mean that particular resource hasn't yet been indexed.
Revisit search engines often because their indexes are updated at regular intervals.
Internet addresses (URLs) change without notice. A URL that was working when a search engine robot/spider previously visited a site may end up being broken when you attempt to visit the same site.
Owners of some web sites do not want their site to be indexed and work to exclude some or all search engine robots/spiders. A polite search engine will respect the wishes of the site's owner and ignore sites such as these.