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The Yodeck Playbox (based on Raspberry Pi) automatically detects on start-up the "preferred" resolution of the connected monitor (usually, its native resolution). Sometimes, the resolution might not be detected properly. This might happen if you power up your Playbox without a screen connected, or if after a power failure the Playbox starts before the screen has the time to power up and report properly. In this case, the standard VGA resolution of 640x480 is used.
Try rebooting your Playbox, it might be just fine. Also, you can follow this guide here to select the desired resolution of your player automatically on boot.
For Advanced, Custom and 4K resolutions we strongly recommend reading this article first.