If you filed in early September, a three month wait is not unusual. If you haven't received an interview notice by mid-January, follow up with USCIS again.
Are you applying for citizenship, or for a green card? The N400 is the naturalization application, but then you referenced a priority date, which typically applies to someone adjusting status to permanent residency in a preference category.
This could happen for any number of reasons, even something as simple as they now have an officer training scheduled on the day of your appointment and now you need to be rescheduled, it could really be anything. I would wait another month or so and if you still have not heard anything, make an INFOPASS appointment to inquire about your case.