Your package's tracking can't be updated until it clears US customs and is scanned by USPS. This time of year there is a huge holiday backlog of parcels. Most likely your item is sitting in a warehouse in San Francisco waiting to clear customs.

Last year, I saw a significant slowdown in US-bound mail, starting in early November, and not really improving until mid to late December. Average time for this type of package was 16 days, but a significant number took 30-45 days, some even 60 days or more.

The long & the short of it is that your package is probably not lost, just slow. If your buyer protection period is about to expire, ask your seller to extend it. Otherwise, keep waiting.