An event is missing
Confirm the calendar source is visible, the event falls within the current view and the correct external account is connected. Then choose the provider sync action again.
Authorization expired
Open Settings and reconnect the provider. Reconnection replaces the stored access credentials without deleting the external calendar.
The wrong account is connected
Disconnect the provider, then connect again and select the intended Google or Microsoft account.
Avoid duplicates
Allow a sync to finish before repeating it. Edit the existing synchronized event rather than creating a second copy in both systems.