Alt Mode video works by creating a direct pipeline from the host device's graphics processor through USB-C to the dock which allows it to output video to connected display. This relies on a few key factors to function correctly, including the host device's graphics processor, the host device's USB-C port, and the cable used to connect the dock.

It's recommended to check these possible causes if the Alt Mode video output on your dock is not working:

• USB-C Port Limitations: Not all USB-C ports support video output (Alt Mode). Check your host device's specifications to confirm it supports Alt Mode video output.
• Incompatible USB-C Cable: Some USB-C cables support power and data, but not Alt Mode video. Use the USB-C cable that was originally included in the box with your dock to ensure it supports Alt Mode video output. 
• Incorrect Host Connection: Ensure you’re connecting to the correct USB-C port on the dock, typically labeled a laptop icon. Other USB-C ports on the dock may not function for connecting the dock to a host device, and using them could result in no function.
• Outdated Drivers: Ensure your operating system and graphics drivers are up to date. USB-C Alt Mode primarily relies on the laptops graphics processor to produce video, so outdated drivers can often cause issues.