Our Drupal 7.50 website is set up so that users can subscribe through CiviMemberships via a contribute page and then we use CiviMember Role Sync Module to assign them a new Drupal role. This has worked well and thousands have gone through this process. However, we have one user who went through the process of creating a Drupal account and then subscribing (becoming a member) and the sync refuses to sync his accounts. I had to give him the Drupal role manually. Names and email address are the same in his Drupal account and his CiviCRM contact record. The contact record shows his membership and contribution. Any ideas how to troubleshoot or manually sync this one user?
CiviCRM 4.7.15 Drupal 7.50 Commons