Hard to say without looking at the site itself. If it's the first site in your sig, I'd say you need more backlinks to get the site properly indexed.
Google won't usually tell you what's wrong with your site. However, if you submit a sitemap of it at Google Webmaster Central:
http://www.google.com/webmasters/
You might get some sort of indication as to what's wrong. Sometimes Google will even send an email as to a reason for a ban if you are registered there.