Office 365 - Outlook 2016 Search not working

We have a brand new iMac with Office 365 subscription including Office 2016. On the mac are 2 user accounts so that they can share the desk in the office. The first configured user (A) has no issues with anything and everything seems to work fine including all search functionality. The secondary User (B) cannot use the mac search functionality in Outlook or Spotlight to search mail prior to the installation date. The mailboxes were in existence prior to the arrival of the mac and search correctly in the Office 365 web client. Is there anything I can change to allow User B to be able to use the search capability in Outlook 2016?

In addition to this, we have another mac (User C) using the same Office 2016 and 365 package and theirs also works perfectly.

I have looked online at various topics where issues are similar but it seems that having 2 user accounts on the one machine is very specific and none of the standard fixes have helped in any way.

Hope someone can shed some light on this.