Search engines are fairly efficient at indexing web pages and displaying them in their search results. However none of the search engines will ever get to index your site if they don’t know of its existence.
There is a misconception that search engines, particularly Google, are all powerful and know everything about every web site – nothing could be further from the truth. There are probably millions of pages that have never seen the tracks of search engine spider simply because no one told them their address.
The search engines learn about your address in one of two ways, either you tell them, or someone else does in the form of a link and that can be difficult if no-one else knows of your existence.
One easy way to tell the search engines of your existence is by creating a sitemap and submitting it to all the search engines. Google makes it easier through their Webmaster Tools sitemap submission service.
XML sitemaps are now the standard. They provide a clear path to all of the pages that make up your website and can include information such as when a page was last updated and how often the spiders should update their information on a particular page.
If you don’t have a sitemap, create one and submit to all the search engines.