After having 2 mobos working well with 6 GPUs each, i decided to buy 3 more of the same model (Asrock H81 Pro BTC R2.0) and got stuck being able to run only 5 GPUs per mobo, i've tried many tricks including using PCI-E card instead of the onboard, disabled audio, serial devices and anything else, changing risers, usb cables, power cables, dedicated psu for GPUs ... etc

I'm always having the last PCI-E port not working (the one on the edge of the mobo).

Did anyone of you get the same issue ?