Why do some events sync and others don't?

I have an iMac 27" running OSX 10.6.8 and a latest model iPod Touch which I have syncing with the iMac and is running iOS 6.1.2.

I first noticed that they weren't syncing reliably when I lined up some Apple Store classes.

I DLed the iCal events and double clicked on them. They show up in the desktop iCal but some do and some don't on the iPod Touch.

The same happens with events I create myself. Some Sync and some don't. I did these all in the same session, so can't understand what is the difference.

I have all Callendars checked in the iPod's Calendars app.

I have tried restarting both the desktop and Mac apps and even restarting both devices completely.

The missing events just stubbornly refuse to show up.

Can anyone else think what else to try.

To be frank looking at the whole process of importing events from the Apple Store, down to the adding of my own events and then syncing them is a tedious shambles. Certainly not something that "just works".