Then I assume it only scans those frequencies which are listed for your region (and cable operator).
These frequency lists are located in the program folder of Plex.
On Windows you can find them in C:\Program Files (x86)\Plex\Plex Media Server\Resources\Tuner\Shared\scanners
I don’t know the exact location on Linux, look out for the folder hierarchy of Resources\Tuner\Shared\scanners
You should then dive down in the appropriate subfolder, which depends on whether you are using the terrestrial or the cable tuner in your device.
(Which brings me to a different idea: is the “full” channel list in the HDHR tv app a combination of the channels which are available via antenna and cable?
Plex might only be able to use one out of these two.)
Then you need to find the .ini file which is used to scan for channels. There is unfortunately no index to find the right one. You need to open them up in a text editor and read the first 3 lines.
When you find the right one, you can try to verify if it’s listing all transponder frequencies which are relevant to you (and add those which might be missing).
See Plex DVR setup - DVB-C Europe: Full frequency range for details.
