1. What version of the Mac OS are you running (Apple > About this Mac)?
2. What version of the iOS are you running (Settings > About)?
3. Do you use MobileMe/ iCloud or another server based sync solution like Google or Yahoo?
4. Do other changes to synced information like Address Book content sync successfully back and forth?
Based on your description it sounds like you have a 1 way sync issue. Events sync fine between the iOS devices and fine from the computer to the iOS devices but not from the iOS devices to the computer.
- Backup your computer and iOS devices before doing anything else:
- Ensure all the devices in use are fully up to date: Apple > Software Update / Settings > General > Software Update
- Make separate backups of critical data:
- Backup your computer Addressbook: http://docs.info.apple.com/article.html?path=AddressBook/4.0/en/ad961.html
- Backup your computer iCal: http://support.apple.com/kb/HT2966
- Reset syncing on your Mac: http://support.apple.com/kb/TS1627
Reply back if that does not resolve your issue.
Hope that helps.