If the virtual remote is not changing channels, you would want to try a few different things. You will want to make sure that the IR Blaster is plugged in and sitting in front of the sensor on the cable box, make sure that the cable box is in IR (Infrared Mode) and not RF (Radio Frequency), and you can also try reconfiguring your cable box settings.
- Slingbox.com - Advanced IR Emitter or Onscreen Remote Control Troubleshooting
- Slingbox.com - Set-top box not responding to remote commands
- Slingbox.com - IR Emitter/Remote Control Troubleshooting for Slingbox 350/500
Hope this helps,