I know it may sound simple, but have you tried powering on your cable box after connecting by using the virtual remote?
Beyond that, here are a couple of articles from our support site with some additional information:
- Watch on Slingbox.com
- Fixing "No video signal" problems with Setup on Slingbox.com (I know this says "Setup" but the troubleshooting will be the same for "Watch")
Hope this helps!
The Sling Moderation Team