Shared mailboxes not showing up auto?

Hi all,

I'm having some issues sharing mailboxes to users so it will auto map in users Outlook software. All users have full permission however it will only show up auto for some users while I need to manually add it to others using these steps in Outlook.

(1). Go to File> Account Settings;
(2). Double click the email address;
(3). In the pop-up window, click More Settings;
(4). Choose advanced and click Add;
(5). In the pop-up window, input the email address of the shared mailbox and clickOK;

Any idea why it works for some and require manual work on others?

Thank you for your time.
Best regards,
Saman