Facebook login with Wordpress Site

I am working on one of the site that is built in wordpress and facing one issue that as per the plugin when we logged in with facebook to the site a user of type author is created by default but in few users a user is not created.In case of other users it is working fine but Few users it is creating the Issue.Below is site Url:-

http://maggcom.com/

On click of Add Story that Facebook login functionality can be achieved.Can you please help me out.

Thanks Mitesh Shah