Being able to ping something is only part of what DFM uses to determine a device is reachable. DFM must also be able to successfully poll the device via SNMP.

Also note that an alert will not clear until all of the events within the alert have cleared. New events may get rolled into an alert all the time. You should go into DFM Fault History and search for events which have cleared.

If you find events are clearing, but you are not receiving notifications for those cleared events, it could be that you have not properly defined your Notification Group. In order for cleared events to go out as notifications, you must check both the Informational event severity box as well as the Cleared event status box.