Google can not connect to the server? Google Webmaster's tool tells me on a site that "Request timed out or site is blocking Google". On that site. I am wondering how to fix this.

Probably relevant info: I'm using Varnish in front of nginx and also user use iptables and DDoS deflate. The site is large but works pretty smoothly, with load less than 2 most of the times. The standard Pressflow 6 robots.txt is there and accessible through browser. 

I am not quite sure how to figure out what blocks googlebots and how to fix the problem. So appreciate your hints.
Check the settings in the nodewords (or another meta tags) module look for noindex