Set Regarding Button Missing on a specific Calendar

Hi All,

Im not a Dynamics Guru we look after the infrastructure for a business and they keep reporting this software issue which is urgent.

The Set Regarding button which shows up on the Outlook Ribbon is missing when any staff member opens up the Calendar for a Specific staff member (Rob).

I have full access to the CRM admin management and have compared accounts with other users that are working with same Security Role access as Robs account.

Please lead me in the right direction what could be causing this set regarding to go missing.

We are using outlook 2013 32 / 64 bit  with CRM 2011 plugin.

OS Windows 10 and Windows 7

CRM server is 2011 as well.