This question can be answered by following the below list of suggestions:

1. Check if you have permission to change the display name format
2. Confirm the Group Policy is properly configured
3. Make sure your Active Directory is up to date
4. Ensure that the display name format is configured correctly
5. Verify that the user account has the right display name format configured
6. Check if the user is properly assigned to the right group
7. Make sure the user name has the correct formatting
8. Check if the display name format meets the Active Directory protocol guidelines