Can't speak for every situation, but, the tech that installed mine did not realize that it defaults to 480i/p. You have to go into the setup menu to change it to output in other formats, i.e- 1080i, 720p, and/or other combinations. Hope this helps.
My audio receiver indicates what it is outputting. It can be tricky in that one network affiliate may be broadcasting in DD 5.1, while another may be broadcasting the same program in DD 2.0 As for the DirecTV HD channels, It's not a given and you have to check the receiver to see what's being output.
Additionally, just because a station is broadcasting digitally does not mean that they are broadcasting in HD. One of the most disappointing things about HD is how little is actually offered. Even the HD channels like ESPN and ESPN2 only broadcast a few hours of HD daily, Same goes for the national networks.
Try a better antenna, it can't hurt and could only help. I've found Digital stations much easier to pick up and much more abundant than I'd expected. YMMV. (and I hate that overused acronym, but it applies in this post)