New WordPress users always receive the "admin" username

Hi, I have the WHMCS MU Provisioning plugin set up and everything works great except for one small thing. When a user is created in WordPress, the user's username is always "admin". On the plugin page, it says:

" If a user does not already exist on WordPress as determined by matching the WHMCS Clients email with the WordPress users email, a new user will be created using the WHMCS Client’s email address and the portion before the ‘@’ as the users name."

That means that the WordPress user's username should be taken from his/her email address correct? Is there some sort of configuration option I need to set for the plugin to set WordPress usernames up as the first part of the user's email address?

Thanks