Hi got some problems too with this board, 
I got one rig with 10 gpus (rx580, 560, 2 vega 56) - no problems at all. all gen2, onboard graphic etc..
I mount another rig with 10 vega 64 but only can connect 9, if i installed the 10 gpu, i have no post on poweron..
Any ideia too what could i check?
I changed to 16 gb ram its the same, have a lot of power too..
No problems in risers too,
my solution was to put one vega in my desktop pc.. but its frustating
Regards,
I got one rig with 10 gpus (rx580, 560, 2 vega 56) - no problems at all. all gen2, onboard graphic etc..
I mount another rig with 10 vega 64 but only can connect 9, if i installed the 10 gpu, i have no post on poweron..
Any ideia too what could i check?
I changed to 16 gb ram its the same, have a lot of power too..
No problems in risers too,
my solution was to put one vega in my desktop pc.. but its frustating
Regards,
Also, do you have power connected to all THREE PCIe PWR headers near the PCIe slots? There will be one "Molex" 4-pin PCIE_PWR1 near PCIE1_1, one "Molex" 4-pin PCIE_PWR2 near PCIE7_1 and another SATA Power Connector called SATA_POW1. All three should be powered from (connected to) the same PSU supplying power to your motherboard ATXPWR1.
Just noticed the SATA_POW1, is this required? http://www.asrock.com/support/faq.asp?id=454