It seems that you are using some issues with the Slingbox PRO-HD since it is connected to a DTA (Digital Television Adapter) and you are missing some of the channels.
This issue could be related to the network layout. We recommend you to modify the network layout and follow this setup: Comcast Cable, DTA, Slingbox, TV. This way, you should be able to get all of the available channels, unless the channels are encrypted. Once you get this signal, make sure to apply the detailed scan channels:
Hope this helps!