I have the opposite problem of this:
All my slingbox HD channels work fine when the TV is physically turned on. When my TV is turned off (even though the cable box is still on), certain HD channels no longer work, although for some reason some of the HD channels do work. All of the non-HD channels still work. I tried resetting the system and reconfiguring and it did nothing.
It's very weird. The channel won't be working at all if the TV is off, then, if you turn the TV physically on, the video appears back on the computer screen right away. I can't figure out why it would matter, or even that the slingbox would know, that the TV was physically turned on. I thought all that mattered was the cablebox was on. If anyone could help I'd greatly appreciate it.
Do you have the cable box plugged into your local tv with an hdmi cable? If so, replace the hdmi connection with a component video/audio cables set and you should be good to go. Some cable companies disable all other outputs for video if a connection is made without a feedback signal for power being detected from the hdmi port.