However if you keep encountering the issue you may be best to disable "Shared Folder Caching"
This can be done as follows:
In Outlook
File > Options > Advanced > Send & Receiving
Then In Send/Receive
Highlight All Accounts > Edit > Account Properties
Then Select Advanced and uncheck Download Shared Folders
And click Ok and close all the Windows, this should now bring live data for shared Folders including Calendars when you open them.
Please note this will leave you unable to see any shared folder information when working offline, and may impact performance on slow or busy internet connections.