I know nothing about the Comcast box, but suggest that you temporarily connect it to a TV or monitor. Using the physical remote (or buttons on the box if applicable), try to find the desired channels. If you can't get them, this is obviously not a Slingbox issue. If you do find them, note their numbers and see whether those work properly with Slingbox.
Thanks for posting to our Forums! I'm sorry to hear that you are not getting the complete channel line up when you connect on the Slingbox.
When you connect to the the Slingbox you are displaying all video and audio information that the cable box is streaming. If the cable box is a dedicated cable box, I would take a TV to the cable box. To make sure you are getting the correct channel line up on the TV. If you are unable to get the channels on the TV, there might be an issue with the cable company end.
Hope this helps!
The Sling Media Moderation Team