Forum Moderators: Robert Charlton & goodroi
1. Your site is relatively new (if so, you will need patience)
2. You have few or no links pointing to your pages from other domains (if so, you will need to acquire some).
If either or both of these is true for you, continue to develop and market your site, and acquire links from other sites that are related to your topic. Also you might set up an account with Google Webmaster Tools and submit a Sitemap. Then you can be certain Google at least knows about your URLs, and you can get notices if there are problems in spidering and so on.
Still, Google will do what it chooses when it chooses. A sitemap can help, but it carries no guarantee. If your site is new, be sure to read:
The Google Sandbox [webmasterworld.com] - building trust to get beyond the filters