1. This is the full tracking of your parcel on the China Post EMS site:
2016-12-06 00: 00: 00.0 00:16:00 Guangzhou Post Express Logistics Company International Business Branch Received
2016-12-06 00: 00: 00.0 02:08:00 Guangzhou City to reach the processing center, from Guangzhou Post Express Logistics International Business Branch
2016-12-06 00: 00: 00.0 14:28:00 Guangzhou City to leave the processing center, sent to USJFKF = ISC NEW YORK NY (USPS)
2016-12-06 00: 00: 00.0 18:36:07 Guangzhou Post express logistics company international business branch to leave the processing center, sent to the Guangzhou terminal
2. Did the parcel that you receive came from Guangzhou to JFK. Right now there is some problem with Guangzhou and JFK. I don't know where is the problem in China or in USA.
3. Like you said some parcel that departed later has already arrive. You might want to contact seller and ask him about your parcel. He should be able to call China Post EMS: 11183, to find out the story of your parcel.
4. If by 18 Jan you can not see any new tracking on the USPS site, then just open a dispute.