I would start with making sure the Slingbox is configured for the input that is connected to the cable box. Next, I would try another set of video cables. If the issues is still there, take a look at the articles below. They should help with understanding what is going on:
- Slingbox.com - Using an HDMI video source with the Slingbox 500
- Slingbox.com - I get no video with any SlingPlayer (Watch, Windows, or Mac OS)
- Slingbox.com - I got a new set-top box, and I can't control my TV now
Hope this helps!