This could be solved by one of the bullets below.
A) Make sure your content is suitable for your device with appropriate display rate (16:9 - 4:3) and correct direction.
B) Check the device orientation on the device detail.
Choose Landscape for horizontal screens.
Choose Portrait for vertical screens.
C) Suggested content resolutions for horizontal screens:
1280 x 720 pixels for 720p resolutions on HD TV's
1920 x 1080 pixels for 1080p resolutions on HD TV's
3840 x 2160 pixels for 2160p resolutions on 4K TV's
D) Suggested content resolutions for vertical screens
720 x 1280 pixels for 720p resolutions on HD TV's
1080 x 1920 pixels for 1080p resolutions on HD TV's
2160 x 3840 pixels for 2160p resolutions on 4K TV's
E) Suggested image formats: JPG, JPEG, PNG
F) Suggested video format: MP4