If you find that your events are not syncing between your calendars, your troubleshooting begins in your device's native Apple calendar.
From Apple's Support site:
Things to check first
- Check the system status of iCloud Contacts, Calendars, or Reminders to check for services outages or connection issues.
- If a reminder isn't showing up on some of your devices, make sure that your iPhone and iPad have the latest version of iOS or iPadOS, or that your Mac has the latest version of macOS.
- Make sure that you're signed in to iCloud with the same Apple ID on all of your devices. Then, check that you turned on Contacts, Calendars, and Reminders in your iCloud settings.
- Check the date and time settings on your device to make sure they're correct for your current location.
For more troubleshooting steps, tap here.