There are two extra feature for to get the alert for the incomplete tickets to the Head of the department.

Age report No user alert.

To enable the option Go to → Reports → Subscriptions.

Age Report:

If you enable the age report, the Manager role gets the reports by email for the tickets which are in On Hold or Open status for more than 6 hours.

No User alert –

No user alert is used to make an alert to the Head of the department, if the user is not available to receive the assigned ticket in a respective department.