Are you synch'ing with Outlook on the PC?
The most common "duh" issues are:
- not including the calendar in the sync profile
- not having any entries within the sync window (by default: 1 week in the past to 2 weeks in the future I think)
I have also had problems where sync-ing multiple things - eg: calendar, notes and contacts - has stopped working and I've had to sync one at a time.
A factory reset solved that problem. I don't recall if there was a problem restoring from backup after the reset - most of the stuff I want to keep comes from Outlook so I'm generally not too bothered about restoring.