A while back I noticed that Google Calendar Sync was not correctly synchronising all my calendar entries. While some were okay, others refused to appear. Having spent a couple of hours last night working on it, I have found what I believe to the be the solution.
The problem was either me being too clever, or Google Calendar Sync not being clever enough.
Back when I started using Outlook as my main calendar, I wanted to prevent my work colleagues from viewing my personal appointments. The easiest way to do this was by setting all appointments to be private by default, and then change them if it was a business item. This involved editing the appointment form in the Outlook designer and saving it as a new form (that I called "Private Appointment".
Unfortunately, Google Calendar Sync only tries to synchronise calendar items that are Meetings or Appointments. My custom form was not being synchronised. I've now changed the default form back, but will need to find a way to convert all my private appointments into "normal" appointments.
Once I knew what the problem was, I was able to Google and find that numerous people have had similar problems when they've installed an Outlook add-on such as WebEx or integration with Live Maps.
Now I know this, hopefully I'll be able to get my calendar working properly again and join it up with the rest of my cloud activities.