The issue that you're describing sounds like it could be one of two things. You may need to change the video resolution setting on your set top box or the video may be being blocked by a form of copyright protection called SOC. The first thing you'll want to do is make sure your video resolution is set to 1080i and not 1080p. Here is an article that may help you:
- Slingbox.com - Video resolutions supported by Slingbox 350 & Slingbox 500?
- Slingbox.com - What is video resolution?
If the video resolution is set appropriately and you're still not getting video then the issue may be SOC. I've provided some articles that may help you better understand this issue:
- Slingbox.com - Troubleshooting "No Video Signal" errors during setup
- Slingbox.com - I get no video with any SlingPlayer (Watch, Windows, or Mac OS)
Hope this helps!!