Check that all links between webpages are followed. Maybe you have too many links on each page? If you have any paid links, or reciprocal links, remove them, or no follow them asap. Get backlinks to your deep pages, as well as your home page, and continually add content (and keep pinging this new and old content) That should sort you out! It's not by any means an overnight fix, but it will pay off.
You need an infinit amount of backlinks. Compress your site into a RSS Feed and bookmark every page. Make atleast 2 backlinks for every page and Google will pick up your site for sure! Otherwhy's if this is allowed, check out my signature.
I would suggest doing everything that everyone has said here including pinging, sitemap submission, RSS feed and bookmarking. As much as you can do but I would also like to suggest Tweeting a few of the links and seeing if it helps.
From reading these post I see that there are lots of reasons why Google would only index a small few links form a site map page. To me it makes sense that the robot.txt file may be a factor.