Try looking for a "Enable above 4G decoding" or a similar option in the BIOS. If you can't enable it/can't find it/still does not work it is probably Windows 10 issue (if you are using it) or driver issues.
Yeah i forgot to mention, Windows 10 ^^
I didn't find enable 4G in bios, all i found was Gen 1 or Gen 2 in some options but i don't know what it means.
Have you identified what PCIE slot is not detected? I can possibly see that one slot is faulty, check also your PSU running 6GPUS might be in power shortage for a 1000w powersupply.