Sorry, we'd need a lot more information here. Are you talking about something you're watching over your cable signal that you want to also view through your SHIELD? I assume the SHIELD is hooked up to a different TV? There are ways to show a live cable signal on the SHIELD, but it requires a device like an HD Homerun network cable tuner, and a CableCARD from your cable company.
Generally a SHIELD (or a Roku, or any other TV box) will only stream video from various sources, such as Netflix, or a local storage device. If the file is still being written you won't be able to open it for streaming, not until the file is completed and the proper header information is applied to it.