What are you seeing for various movies? TCMHD usually broadcasts all their movies OAR (Original Aspect Ratio), however, since many of the movies are old, many of them are 4:3 aspect and would have black bars on the sides. Only 16:9 movies would fill your screen. TCM also has a lot of 2.35:1 movies which have black bars above and below the image.
Now, if you have bars above and below, as well as to the sides, then that would indicate it's being incorrectly sent as SD. Have you tried looking at/recording various movies, especially some of the more recent ones?