Do you have a Google webmasters account and have you verified your blog with the three big search engines?
If not, then start start here by verifying your ownership of your blogs with the 3 big search engines http://en.support.wordpress.com/webmaster-tools/
Do you apply basic SEO techniques to your posts and the structure of your blog? It's difficult to attract search engine attention if you don't http://onecoolsitebloggingtips.com/2009/08/04/basic-seo-elements-for-bloggers/
Does your blog contain duplicated content? Please read this http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=66359
WordPress.com blogs are not allowed to be used to drive traffic to third party sites. See: Section 2, 5th bullet http://wordpress.com/tos/
WordPress.com blogs are not aggregation blogs. If you wish to have an aggregator blog then tumblr allows aggregation of 5 feeds to your own content only.
Have any of your posts been linked to by other bloggers who have blogs with a higher PageRank than your own blog has? If so then that will draw search engine attention to the posts in question.
I'm a blogging tips blogger and you will find that I do have posts that are focused on how to gain search engine attention, how to attract targeted readers to your blog, and how to increase your organic (unpaid) traffic to your blog.