This is happened because of several reasons. At that time,
1). The admin fill wrong department for these employees.
2).The admins have to check whether they fill department for them or not.
3). If the admins set Teammate Visible list as 'My Team' in policy setting, this employees can't see others when there is no one who report to them. So. The admins need to set 'All' to see all employees and 'Team only' to see the same department employees.