Why my site not crawled by Google?

Submit sitemap.xml and fetch manually in webmaster tool or problem may be in your meta robots tags.

there are so many factor for search engine didn't crawl if you are not provide any sitemap. if you had not permit to google search engine to crawl site through robotstxt

Why hasn't Google indexed all of the URLs in my sitemap.xml?
My site has a comprehensive sitemap.xml which is being frequently crawled by Google. According to their webmaster tools, only about 80% of the URLs in the sitemap are actually being included in the index.
Should I be worried about this? Is there anything I can do to improve the ratio of indexed pages?