Same issue here. All my calendars are iCloud calendars but I don't see them on my home iMac (using Lion) - also all iCloud calendars - when I enter them onto my iPhone 5. I have downloaded the new iOS 6.1.2. That update was to fix an issue with Exchange but maybe they screwed up the sync of Calendars in iCloud??
If someone could help, that'd be great!
I believe sync is working. I think IOS 6.1.2 caused some new attribute in the event record that Calendar doesn't recognize.
I do not use iCloud (had the same problem with multiple duplicate events which took hours to fix) for calendars or contacts. I sync wirelessly with my MacBook Pro once or twice a day: very painless.
Since I updated my iPhone to IOS 6.1.2, events created on the iPhone do not show up in Calendar. Events created in Calendar do show up on the iPhone.
I also have an iPad which I updated to IOS 6.1.2 over the weekend.
If I create an event on the iPhone, then sync with the Mac, then sync the iPad, the event shows up on the iPad. The only place the event could have come from is the Mac. Calendar still does not show the event.
My guess is that there will be a Calendar update soon. I hope so. It was driving me nuts.
Can't explain WHY but i solved my problem. I first exported my iCal to my desktop, then deleted all my events by deleting my calendar. I then created a new calendar with a new name and synced. All went well. Since I didn't have multiple calendars I had no problems, but you can probably export multiple calendars and create multiple new calendars.