Synchronization of Roles/Groups doesn't work completely

We are using the fantastic phpBB module in Drupal that works well for the synchronisation between Drupal and phpBB when considering groups.

When an user is adding in a group in Drupal, it's OK but if I'm deleting an user from a group (in Drupal or in phpBB), this action is not transmitted to the other system (phpBB or Drupal).

Do you have an explication ?

Thanks.