Google is a mechanized search engine, which employs robots known as ‘spiders’ to crawl the web on a monthly basis and find sites for inclusion in the Google index.
Reasons your site may not be included.
1) Your pages are dynamically generated. Google are able to index dynamically generated pages. However, because the web crawler can easily overwhelm and crash sites serving dynamic content, Google limit the amount of dynamic pages it indexes.
2) You employ doorway pages. Google does not encourage the use of doorway pages. Google want to point users to content pages, not to doorways or splash screens.
3) Your page uses frames. Google supports frames to the extent that it can. Frames tend to cause problems with search engines, bookmarks, emailing links and so on, because frames don’t fit the conceptual model of the web (every page corresponds to a single URL). If a user’s query matches the site as a whole, Google returns the frame set. If a user’s query matches an individual page on the site, Google returns that page. That individual page is not displayed in a frame — because there may be no frame set corresponding to that page.
If you are concerned with the description of your site as seen by search engines, please read “Search Engines and Frames”. It describes the use of the ‘NoFrames’ tag, which is used to provide alternative content. If, instead of providing alternative content, you use wording such as “This site requires the use of frames” or “Upgrade your browser”, then you are excluding both search engines and people who use browsers with frames turned off. (For example, audio web browsers, such as those used in automobiles and by the visually impaired, typically do not deal with frames, which are a visual mechanism.)
Excerpt taken from Google Webmaster Info