Were you streaming videos to TV via network connection, or just using the "play to" (stream to TV) option?
It is expected that the streaming will not use both of the screens to display the video, but leave your PC screen for you to do other stuffs.
If you just want both of screens to play the same video simultaneously, how about project your windows desktop to TV under duplicate screen mode?
Use "[Windows] + P" keyboard hotkeys to switch the display mode of your laptop.
Note that the DRM blu-ray playback is not compatible with this screen copying mode.