1. Check to make sure that the Planner task has a due date assigned to it.
2. Make sure you have the same Microsoft account signed into both Planner and Outlook.
3. Ensure that your Outlook account is syncing with your events.
4. Check if the task’s due date has expired. Existing tasks that have already passed their due date will not sync.
5. Check the garbage or junk folder of your Outlook account since Planner tasks/ events synced are treated as emails.
6. Make sure that the Planner is enabled in Outlook.
7. Try disconnecting and reconnecting the integration of Planner and Outlook.