I'm not sure if the dead area south west of San Antonio is because I can't see it, or because it's just not an area where flights normally track.
I'd say it's because you can't see there. I don't have the fancy scripts and add-ons in this thread, but here's what I see from Wooten (between Burnet Rd and 183 north of Anderson Lane).
First image is using my ground plane antenna in the attic.
Second image is from the 26" FA antenna about 12' above the roof.
Graphed by Planeplotter so does not include MLAT planes with my current setup. I noticed that today there is a small segment south of Louisiana that is about 400 miles away from my location.