Google crawling but not indexing new content on high traffic site

[FreeThemer]

I have a domain which gets 1M+ uniques/mo with a sitemap generated by my CMS systems. Google webmaster tools says it is downloading the sitemap daily but about 95% of the content from the past 10 days is not indexing. The site is putting out about 10 new pages a day, each of which is automatically added to sitemap. There are virtually no crawl errors or robots.txt problems.

I was thinking that invalid HTML was the cause, but one of the articles it picked up in the past few days has plenty of invalid things happening.

Any avenues that I may have overlooked?
Thanks in advance!