As many reviewers have noted, Provider for Google Calendar does not important events from a Google calendar, even when synchronization is manually requested. The only way to get it to sync appears to be to reboot Thunderbird. This has been an issue for a long time. Is the developer listening? Please fix this bug.
Note, in modern version of Thunderbird this add-on is no longer need for syncing events.
Issues can be reported by contacting the Author using the add-on’s Support site, Support E-mail or home page - find your add-on at https://addons.thunderbird.net/thunderbird/