you could try and disassemble the unit and clean the contacts and reseat the connections to see if this works. secondly, i would try and install all updates and drivers associated with the model. thirdly, i would check the settings in bios or os settings to ensure that the functionality is indeed enabled and working properly. with all three of these steps done, you will find an error somewhere or will be in the ballpark of what to troubleshoot. start here for diasassembly and reseating the cables/ cleaning contacts, then proceed with updates and so on:
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post back with any findings for further assistance.