This can be a number of reasons. You need to make sure that you have the following things in place.

Make sure you have a working sitemap that is connected with your Google Webmaster Tools. Use any of the sitemap plugins to generate your sitemap. After this is done go to GWT, choose your site, crawl, sitemaps and select "submit a sitemap".

Make sure your post is set to be indexed. If you are using an SEO plugin like YOAST (my recommendation) and All In One SEO, then the options can be found at the bottom of your posts.

You also need to make sure that your site is actually set to be crawled by the search spiders.

Now that all of these are taken care of lets move on to the next..