http://www.karateforkids.com.au
I’ve discovered it’s related to using reCAPTCHA. Any forms without a reCAPTCHA box will send, but with a reCAPTCHA box (although it will verify “not a bot”) will be flagged as spam when you hit “submit”.
We don’t use the “Comment Blacklist” and there are no other third party cache plugins that I’m aware of.