I came across this weird problem recently, where my Tungle account (tungle.me/laurenwood) was only synchronizing some of the events on my Google calendar. I checked they were set to the right calendar (yes), marked as busy (yes), and still couldn’t find the answer.
This is an issue that quite a few people have had. Apparently what’s going on is that Google calendar supports three types of availability for events: free, busy, and tentative. In the UI on the browser and on the Android they only show the free (available) and busy. BUT, when you create an event on the Android calendar, it’s assigned the “tentative” status. I have no idea why, since I added those events to the calendar myself, they’re not invitations that I still have to accept or decline. And the fact that there appears to be no way to change the status once the event has been created just makes it worse. I tried changing the status manually to “busy” in both the browser and the Android calendar app but that didn’t work. The only thing that did work was to delete the event and recreate on the desktop.
Fortunately tungle implemented a fix — you can choose to mark the supposedly tentative appointments as “busy”. Here’s hoping any other apps I may wish to sync with my Google calendar also implement this feature.