Hi, unfortunately your post does not contain any clear details of your notification that would assist with debugging.
What have you checked\not checked?
How is the notification triggered?
If by a sysevent, then check your event logs to confirm that the event is still triggered with correct parameters, and if not review the script\workflow\flow that generates the event for any changes.
If triggered by conditional values, check to validate that the task is still meeting the trigger conditions.
Check the notification to ensure that Users/Groups in fields values are correct and the notification is still active.
