When you have two-way calendar sync enabled for a calendar, events you create/update/delete in Marvin will be immediately synced to your "remote" calendar. But changes that you make in your remote calendar are only taken over when you press the "Calendar Sync" button at the top-right of your screen, or when an automatic sync takes place.
Automatic calendar sync is on by default when you have calendar sync enabled. Every 30 minutes Marvin checks for updates from your remote calendar, as well as any local changes that didn't make it to your remote calendar. You can turn it off for a device with Automatic sync (on this device) in the Calendar Sync settings. Automatic sync doesn't run in the mobile app.
Automatic calendar sync only happens on one of your devices. Whenever you connect with a device, Marvin checks whether this is the first device that you've opened in the last while. If it is, then it becomes the primary device, and so long as it stays open it will remain the primary device. If you close Marvin on this device, it may take longer than 30 minutes for automatic calendar sync to start up on your other device. You can always use the manual "Calendar Sync" button to grab the latest data if you see there are some changes that haven't been taken over yet.
Note: with two-way sync, changes you make in Marvin are sent to Google Calendar, Outlook and CalDAV calendars, including changes to a single occurrence of a recurring event.
See this guide for outlook calendar troubleshooting.
See this guide for google calendar troubleshooting.