We are seeing intermittent behavior with CiviCRM and Wordpress. Sometimes the membership registration page will create a new Wordpress account as expected. Sometimes it doesn't. What would cause this behavior?
The CiviCRM membership registration page has the Wordpress account creation required checked in the profile and primary e-mail is present on the form. Here is the form for reference: https://aslta.org/membership/selection/
WordPress 4.3.1 CiviCRM 4.6.6