I was looking into possible causes for intermitent calendar syncing across mulitple devices and found an article that outlines how to troubleshoot this type of issue. It is named iCloud: Troubleshooting iCloud Calendar found here http://support.apple.com/kb/TS3999. It is a LOT of information so do take your time as you go through it.
For troubleshooting the iMac,
- 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.
And for troubleshooting on iOS devices,
If you are having trouble with the Calendar app on an iOS device (iPhone, iPad, or iPod touch) and iCloud Calendar, try each of these steps, testing after each to see if the issue is resolved:
- Make sure you've updated to iOS 5 or later and that your iCloud account is configured correctly in Settings. For instructions on setting up the iOS for iCloud, see iCloud Setup.
- Make sure your iOS device is online. Attempt to view www.apple.com and iCloud.com in Safari. If you can't connect to the Internet, your iCloud calendars and events will not update in your Calendar application. Click here for more information about troubleshooting your internet connection.
- Open a secure website from your iOS device to test if you are online as is necessary for iCloud Calendar. This also tests if ports 80 and 443 are accessible. Calendar requires port 443 access to iCloud in order to push and pull updates to and from the iCloud Calendar servers.
- If recent changes simply don't appear in Calendar, try forcing a refresh by tapping Calendars in the top-left corner of Calendar to load the Calendars list. Then, tap the Refresh button in the lower-left corner of the Calendars list window.
- If you add new events on your iOS device, but they do not move to iCloud or other devices, you may want to verify your Default Calendar settings. If you would like to create new events in a specific iCloud calendar, then set that calendar to be your Default Calendar. You can modify the Default Calendar settings in iOS at Settings > Mail, Contacts, Calendars > Default Calendar.
- Quit and restart the Calendar app on your iOS device:
- Press the Home button.
- Double-press the Home button to open the multitasking menu.
- Tap and hold the Calendar icon until it jiggles.
- Tap the "X" to remove it from the list.
- Press the Home button to get back to your Home screen. Wait a minute before reopening the Calendar app.
- Restart your iOS device by holding down the Sleep/Wake button and then swiping the screen when prompted to Turn Off. This may sound simple, but it does reinitialize your network and application settings and can frequently resolve issues.
- Change the Sync settings for your calendars:
- Tap Settings > Mail, Contacts, Calendars.
- Scroll down to the bottom.
- Tap Calendars > Sync.
- If sync is set to All Events, set it to a specific duration, such as Events 1 Month Back. If it's set to a specific duration, set to All Events.
- Wait several minutes and try Refreshing Calendar again.
- Turn iCloud Calendar off and back on:
- Tap Settings > iCloud.
- Tap the Calendar slider off. Choose to delete data only if your data exists at icloud.com/calendar and/or one or more of your computers. Otherwise choose Keep Data.
- Wait a few minutes before turning the Calendar slider back on.
My iPads, iPod Touch, and iPhone 5 were not sharing calendar events. The last suggestion in Sterling's list above, to turn the iCloud Calendar off, then back on, seemed to do the trick. When shutting off the iCloud Calendar, I chose to retain my calendar data on the device, then remerged to iCloud after turning the calendar back on.