I'm not an expert on this stuff, but it might be because google views some of the pages as duplicate content or considers the pages low quality. Are there many people linking to these pages? Have you tried submitting the pages to google (via the webmaster tools or whatever it's called)?
You'd be better to post this question in the open forum, I don't think it really needs to be on the inside.
You'd be better to post this question in the open forum, I don't think it really needs to be on the inside.