Quick question what other motherboards are confirmed to work with 7 gpus, because I keep having problems with the Asus Prime Z270-A.What boards are you using and how many GPUs?
I have a Z270-A (BIOS v 0906) that runs 7 1070 GPUs without any problems. OP says you can run 9 on this board if you use the m.2 to PCIE adapters.
I also have a Z270-AR rig. For anyone considering buying one I can confirm it works with nvOC. The only difference I can see vs the -A version is the -AR lacks a displayport output and DVI-D output. Everything else is the same.
It wouldn't hurt to try BIOS update if you haven't already (start with a freshly imaged thumb drive after applying the BIOS update), and re-seat all your GPU/riser connections. Ensure you only have 2 risers per single SATA/molex cable from PSU (most cables allow 3 devices per cable). If you still have 1 GPU down at that point I would power down and swap around risers and their associated cables/adapters 1 and a time and rebooting to see if something is faulty.