If you were using a home HD receiver, you might find improvement in using a different type of antenna, especially an outdoor directional antenna. Even good indoor antennas available at many retailers provide much better performance than the typical wire antennas included with these units.
Receiver sensitivity is also another issue. I've noticed much better sensitivity from the Sangean and Accurian (Radio Shack) models than I have the Boston Acoustics. Of course, that's another topic within itself, which I'm sure has been discussed in the HD Radio board here.
Out of curiosity, and if you don't mind me asking, what radio are you using and what type of antenna?