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Any number of things could cause this issue, so I would recommend this article named iCloud: Troubleshooting iCloud Calendar found here http://support.apple.com/kb/TS3999.
These sections in particular:
- Make sure there are no current issues listed on the System Status that affect iCloud Calendar. The System Status is located on the iCloud Support webpage.
- Make sure you have fully enabled iCloud Calendar for the client you are using (for more setup and troubleshooting information on iOS, OS X, Windows and iCloud.com, see the additional details below).
- Check to see if the issue you are seeing is covered in iCloud Help. To search iCloud Help, visit help.apple.com/icloud.
- Check your settings and preferences as something may not be set up correctly. For example, having incorrect date and time settings can cause issues which are easily resolved by entering the appropriate settings.
Troubleshooting Calendar on OS X Mountain Lion (iCal on OS X Lion)Note: For instructions on setting up iCloud Calendar with Calendar or iCal on your Mac, see iCloud Setup.If you are having trouble with Calendar or iCal on a Mac and you are using iCloud Calendar, try each of these steps, testing after each to see if the issue is resolved:
- Make sure you are using OS X Lion v10.7.2 or later. Click here for help verifying the version of OS X you are using.
- Make sure your computer is online. Attempt to view www.apple.com and iCloud.com. If you can't connect to the Internet, your iCloud calendars and events will not update in iCal. Click here for more information about troubleshooting your Internet connection.
- Verify that your iCloud member name is entered into the iCloud Preference pane in System Preferences. See iCloud Setup for more information about setting up iCloud on a Mac.
- Make sure you enter and update calendar information in the iCloud section of Calendar or iCal. Only calendars and events in the iCloud section of the Calendar or iCal Calendar list will be pushed to iCloud. Both Calendar and iCal for the Mac support the display of and interaction with multiple accounts and datastores, for example, iCloud, On My Mac, Exchange, Google, and so on. This makes it easy to create or modify events or calendars in a section of Calendar or iCal that may not be updating iCloud. Learn more about moving local calendars to iCloud.
- In some cases, you may find that you have data duplicated in both the local On My Mac and the iCloud sections of Calendar or iCal. Learn more about resolving this issue.
- You can make the iCloud calendar your default calendar for all new events. In Calendar on OS X Mountain Lion, choose Calendar > Preferences > General, or in OS X Lion, choose iCal > Preferences > Generaland under the Default Calendar heading, select one of the Calendars under the iCloud heading. Close the Preferences window to save this change.
- If recent changes simply don't appear, try forcing Calendar or iCal to refresh by selecting Refresh All from theCalendar menu.
- Make sure your account was successfully created in Calendar on your Mac (or iCal). After upgrading to iCloud Calendar, you may need to quit and reopen Calendar (or iCal). To confirm your account is configured correctly in iCal:
- Open Calendar (or iCal).
- Open Calendar (or iCal) Preferences.
- Click Accounts. You should see an account with your iCloud member name entered.
- Turn iCloud Calendars off and back on
- Quit Calendar (or iCal).
- Choose Apple () menu > System Preferences > iCloud.
- Remove the checkmark in the checkbox next to Calendars.
- Close System Preferences and wait about a minute.
- Open System Preferences and choose iCloud.
- Replace the checkmark next to Calendars.
- Close System Preferences.
- Open Calendar (or iCal) and test to see if the issue has been resolved.
- Restart your computer. This may sound simple, but it does reinitialize your network and application settings and can frequently resolve issues.