Set up an appt with Comcast and have their technicians check your line; it's a no cost visit. The signal interference can stem from loose connectors found behind the cable boxes, at the wall plate, at the splitters, or they can be a combination of kinked or pinned cable wires. Sometimes it can even be the cable behind the wall plate. So, the sooner you schedule the appt with them, the sooner they can remove the filter that is preventing you from accessing your OnDemand content. In the meantime, check and tighten any loose connectors.
it's a no cost visit.
If the problem turns out to be on the customer's side of the demarcation point / grounding block, then there will be a charge for the tech visit and any needed hardware.