Using New Outlook on Windows for personal email, calendar, and contact management
If it used to work and now doesn't, remove the account from new outlook. If asked to remove it from all devices, say yes. Then add it back.
When the accounts stop syncing, it usually means the cache is corrupt on the server. Removing the account will fix it. It you use more than one Outlook app with the account, you need to remove from all devices to clear the cache from the Microsoft cloud server.