If you are using component cables to view the Slingbox, it may be a bad cable. If you are connected to the cable box with HDMI, you may be running into HDCP and would need to run component cables to view the Slingbox properly.
Here are a few articles that reference:
When I connect my set-top box to my TV with HDMI connections, I can't stream video at all using any version of SlingPlayer. Why?
Sling Media Moderation Team