Facebook Comments Only feature isn't working

Hi, I'm launching a new blog and I'm trying to get to grips with Ultimate Facebook. I've created a facebook app with the ID and secret code which has shown as 'green' within the plugin so that has connected correctly.

I've checked the Comments option to make facebook comments be shown and also checked the box to make the facebook comments the ONLY comments shown.. in essence to get rid of those wordpress comments.

System: Wordpress: 3.31
Thene: Twenty Eleven 1.2
Ulitmate Facebook Plugin: 2.0.1

The ability to ONLY have facebook comments is not working on my website.

Can you advise what I can do to help fix this please? I'll soon be trying to implement this on my clients website so I'm testing it on my new blog first.

Thanks
Cathie.