Some of the built-in macOS applications have the possibility to activate a ‘special’ debug menu. One of these applications is Reminders, the debug menu gives you the option to manually Update/Refresh with your iCloud account.
Quit the Reminders application and start a Terminal window, apply the following command:
defaults write com.apple.reminders RemindersDebugMenu -boolean true
After applying to command start the Reminders application, the Debug menu will appear in the Reminders menubar and gives you the option to manually sync with iCloud whenever you want.
To disable the debug menu, quit Reminders and type the following:
defaults delete com.apple.reminders RemindersDebugMenu
Start de Reminders application and the debug option is gone…