I would recommend in checking the respective pass-through connectors from the Slingbox which are connected to your set-top box. It is possible that the set-top box isn't providing a video signal. The Slingbox can only rebroadcast what it "sees" and "hears", so that would be a good place to check first.
A common issue is the use of a HDMI connection on the set-top box. Some set-top boxes employ High-bandwidth Digital Content Protection (HDCP) standards which either shut off or disrupt the analog outputs when ever HDMI is used.