Just a hunch:

Your TV may be for the Korean TV market and Korean HDTV might broadcast on different frequencies than USA HDTV.

If you bought a Korean model, chances are good that it tunes the DVB-T digital channels used in South Korea, not the ATSC digital signals use in North America. If you have an HDMI connector or component connectors on the back of your TV, chances are good that you can buy an ATSC HD tuner and connect it to your TV set. eBay has a number of HDTV tuners available.