What I understand from your post, is that you can't get all the channels using coaxial cable only.
This might happen because the strength of the coaxial signal, or that the channels are encrypted. What I would recommend to you, is to use the Analog Cable option to configure it and to discard that as a potential issue..
You can also check the following link.
I know this is an old forum thread, but if the slingbox not told that it's on a cable tv line, and is told it's on an antenna connection, it will only see up to channel 13. I have a slingbox classic on comcast cable on analog, it tunes up to channel 90-something. This is the same issue as if you had an older analog cable-ready tv and had it set to air/antenna rather than cable in the settings. Recheck settings and rescan, it should pick up any channels above 13 that can be picked up on an analog set connected to cable.