I found this solution in another discussion. It's from this website: http://iphonewriter.blogspot.co.nz/2008/05/how-to-fixing-ical-iphone-sync-issues .html)
"You'll know the syncing issue when you see it, because the new events you enter into the Calendar on your Apple iPhone will not appear in iCal. Said another way, you'll enter a new event into your iPhone, use iTunes to sync with your Mac (or MacBook in our case) and the new event on your iPhone will not show up in iCal. Keep trying, synch as you might, iCal and the Apple iPhone just won't get along.
Here's the down and dirty fix:
- Open iCal and backup or export your entries. Make a note of the fie name and location, you're going to need them in a minute. Once your backup/export is completed, close the iCal application.
- Open Finder and remove everything inside the "Username/Library/Calendars" folder. For instance, if your username is "Joe", then move everything inside the "Joe/Library/Calendars" folder.
- Open the iSync application. It's located in the "/Applications/" folder. Once iSync is opened, go into the iSync Preferences (iSync -> Preferences) and push/click the "Reset Sync History" button. Then, close the iSync application.
- Re-Open the iCal application and Restore your calendar. When prompted, use your notes from Step #1 to select the file your created earlier.
At this point all my appointments were showing up properly in iCal, and did not disappear after syncing again with the iPhone.