Admins unable to reassign /assign ticket to some of users and/or Assignee drop down list does not displays any users
The Access Right to show standard users that are members of any scope is disabled .
Please check and verify the Role assign to your Admins/Users if they are allowed to see the standard users that are members of the same scope.
Navigate to User Security > User Role > select the Access Rights tabs > click edit and navigate to Ticketing > Manage Tickets > (Show Standard Users that are members of any Scope)
All VSA version