Siri is installed on the iPad 3 keeops disappearing

Siri was originally it was available on the iPad 3, but it has disappeared. I have looked in settings under "General" and it is not listed as an option. I have reset it back to it's factory setting several times and Siri is initially available, but then disappears. I made sure the device is hooked up to WiFi. When I look under Restrictions in the Settings, Siri is grayed out. I have tried enbling and disabling the restrictions, but it does not seem to make a difference.