This will resolve the issue going forward, for newly created users. If you still have an issue:
1. In Administration, select Set up system security, open the user who does not show as an approver.
2. Change security settings radio button from Supervisor to Selected Group Rights.
3. Select a group and add to the Member of list.
4. Save and close the user.
5. Re open the user and change rights back to Supervisor rights.
6. Save and close the user and they will show to be an approver.