This could due one of the following reasons:
Adding the assignment by a manager, an admin or a user with escalated access rights.
The authorisation email is received in the Spam or Junk box.
Having a conflict in authorisation rules for assignments.
If none of the previous reasons help, raise a new case online and reference the title of this article providing the following details:
The name of the employee who has the assignment in their record.
The date of the assignment.
grant our team access to your system.