Goto Admin Panel -> Settings.
Scroll down to "Autoresponders (Global Setting)"
Is "New Ticket:" enabled?
Also click on the "Departments" tab.
Click on the departments you have configured.
Ensure that "New Ticket:" is enabled.
Have you made any changes to OST 1.6 from the stock installation?
Have you checked the PHP logs to see if its throwing an error?
Have you checked the MySQL logs to see if the query is generating an error?