Do you have TiVo boxes or V6? I would assume V6, but that's a fast way to give wrong advice - check here if you're not sure.
Assuming V6... how is the affected box connected to your homehub (For it is this that provides the internet connection) - is it an Ethernet cable or WiFi? If youj follow Home > Help & Settings > Settings > Network, the screen will report your connection status, and if applicable your WiFi signal strength. What is shown?
Do you get any error messages when trying to use OnDemand, this would help diagnose what your issue is, too.
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