Outlook calendar sync issues

Hello

My users are experiencing outlook calendar sync issues, when assistants update the meeting details but doesn't get updated for the boss. 

I have tried all possible troubleshooting options,

1 - cache / online 

2 - installed / reinstalled office

3 - updated office

4 - adding / removing the calendar many times

I haven't used any of calendar command line switches, don't know which one is the right one. 

Pl can someone provide some help

thanks