This is a great presentation from Google to better explain how difficult it is for a search engine to crawl and index basically an infinite amount of data available on the Internet. If you make it easier for Google (which has 65-70% of all search traffic) to crawl and index your website and associated content, then you will increase the likelihood (nothing is guaranteed) that your website and content will be crawled and indexed correctly.

Find additional support and help from Google here – Google Webmaster Tools