There could be a few reasons why a URL is not indexed by Google:

• Google cannot find the URL on its own (i.e. there are no public links to it)
• Google cannot access the link (due to authentication or another block such as a 4xx or 5xx error or a firewall/bot block)
• Google has been asked not to index the page (via robots.txt or meta tag noindex)
• The webmaster has asked Google to remove the page from the index (they typically only do that for non-accessible or blocked pages so it would be in conjunction with one of the items above)
• Google didn’t get around to crawling the page yet (if it’s new for a example or very deeply nested in the navigation)
• Google crawled the page but decided to drop it from the index. Google doesn’t keep every page in its index. If it’s found to be duplicative or low value, Google may decide to discard the page from its index.
• It may be indexed but may not be in the index of the data center serving your search yet. Google has slightly inconsistent indices depending on which data center serves your search results. These will even out over time.

Google Webmaster Tools will allow you to inspect the index and crawl activity and see if there are any errors. You can do a Google search using:

site:mydomain.com/url/path

to see if a site, directory or page is indexed