i do not have a sitemap. however, i had a forum once without a sitemap, but possibly all of its threads were in google. i now have a blog that google is hardly going through. the site structure is a bit different than my forum. on the front page, i have about 4 article summaries that all link to the full articles. the front page also has a "next" link that lets readers go to the next page to view the summary of the next article. that page also has a link to the full article. that's the way all of the articles are displayed. i am also using mod rewrites. do you think my site structure and use of rewrites is causing google to not add most of my entries?
a sitemap can definitely help google schedule itself to crawl your entire site. more external links helps search engines pick up the site quicker too. If your internal links are well covering all pages than it should be quite ok for search engines to find them all in time. not sure if your mod rewrites are having one unique URL pointing to one article, blogs tends to have different URLs (from archive, tags etc.) pointing to the same page and that can cause duplicate content issue.
Better to create a sitemap and have a sitemap.xml page. The codes for the sitemap.xml can be developed free of cost (from www.freesitemapgenerator.com). Sign in to the google webmaster tool and submit your site sitemap. I hope the problem will be solved. Hoping to hear from you