to use 6+ GPUs on Windows you should -
 
1) turn on Above 4G Decoding in UEFI settings
2) your hard disk drive should use GPT partition scheme
3) and you should boot Windows in UEFI mode (BIOS/CSM/Legacy mode won't work and WILL cause issues exactly like yours when mining with more than 5 cards).

more info about converting your Windows installation to GPT/UEFI boot mode here -
https://www.windowscentral.com/how-convert-mbr-disk-gpt-move-bios-uefi-windows-10