It could be for a number of reasons, but it's best to start with basics. Check your links on your main page to the other pages with sim spider: [searchengineworld.com...] Make sure the links are crawlable.
If they have that PR, they must be indexed by Google...
What happens if you put up a new page linked from the home page? Does it get crawled soon after the index/default gets its regular crawl? (I have pages a few clicks away from the home-page that only get crawled every 3-5 weeks).
If your inner pages are indexed, but just not getting crawled that often, you could play with the internal navigation from the home page and steer bots towards those pages that you want crawled frequently. That would be ones you change often, I would guess.
ADDED: Those pages not getting crawled often are indexed right? If you do a search of, "www.site.org/infrequently_crawled_page.htm", do you find them?