Thank you for using our online answers forum. Sorry to hear you are having issues getting video on your Slingbox Pro. There are a few things we can try to troubleshoot your issue. Have you tried eliminating the Slingbox as a pass through (I.E. hook the cable box to the Slingbox and also hook the cable box directly to the T.V.)? Have you tried using a set of component cables from the cable box to the Slingbox?
Here is a link to help troubleshoot your issue further!
Hope this helps, feel free to reply back with any other questions or concerns you might have.
The Sling Moderation Team