good day folks

having a problem with a rig i'm  building, rig consist of

mobo- GA-p55a-ud3r
Memory - 16gb
HDD - 400gb
Cpu - Core i5 280 GHz

Gpu 0 - R9 280x (in PCIe  slot 16)
Gpu 1 - R9 280x (in PCIe slot 16)
Gpu 2 - Rx 570   ( uses PCi riser card in pci 1 slot via usb)

My problem it that the machine doesn't pic up Gpu 2(RX570)

is there something that i'm missing, any help will be appreciated.

thanks