Yes, that's how it works in US. Very slow but steady. I do understand you, because I had the exactly same experience. Don't worry, just be prepared for the interview, and pack your gear. btw, did they say that they issued it or sent it? If they said they "sent" it, it would be delivered in a week. Keep calling, sending email and DO keep a note of the staffs' name and contact date. 

네 그렇게 하고있구요 지금 3명정도에게 멜보내고있구요 1명이 12월1일에 프린트 됬다고하고 또 다른한명은 이번주에 보낸다고 했는데 확인해보니 아직이네여 또 전하해서 닥닥할려구요 방법이없습니다