I got 6 running with simple miner, I could only get 4 out of 6 running with win 10. Were you able to solve the problem?
There's a few things that can go wrong when setting things up:
1) Bad risers. Be sure to order +20% more risers than you actually need. I generally see about a 20% failure incidence across risers. Also, when replacing a riser be sure to replace the whole thing, not just the PCB. Sometimes (rarely) the cables are bad.
2) Drivers not installing. On a few of my builds, the drivers did not install properly automatically. This can be solved by opening your Device Manager and looking for devices that pop up as having no drivers installed. I've seen GPUs listed as audio devices, video devices, etc. Then right click and manually install the driver. The way to validate that the rogue device is in fact a GPU is to remove the offending GPUs, reboot and see if the rogue devices are still listed.
3) Bad motherboard. Personally, across all of my builds, I have had no QC issues with H81 Pro BTC+. I would research the above 2 incidences heavily before considering this option.