Sometimes computer manufacturers limit the video memory the system will use which is done at BIOS level. Sometimes that option is hidden in the BIOS to avoid changes by the consumer.
Our processors come with something that is called Dynamic Video Memory Technology (DVMT) in which the graphics controller will automatically adjust the video memory.
I recommend getting in contact with the laptop manufacturer in order to check if the video memory can be manually adjusted or not. We do not recommend setting video memory manually due to the system may not work properly.