Why is my computer not noticing the graphics cards I've plugged into it?

Question

I am attempting to connect a couple of "Radeon RX 480" graphics cards to my BIOSTAR TB85 motherboard via PCIe risers.

I have installed Ubuntu 16.04.3 LTS onto the machine, plugged the graphics cards into the PCIe ports via risers, and followed AMD's instructions to install the graphics card driver amdgpu-pro.

However, my machine does not appear to have any knowledge of the graphics cards' presence there. For one thing, the clinfo program does not appear to notice the cards; the output of running $ clinfo is

$ clinfo
Number of platforms                               0

For another, the lspci program does not appear to notice the cards either; the output of $ lspci is the same whether I plug in or unplug the card PCIe risers.

$ lspci
00:00.0 Host bridge: Intel Corporation 4th Gen Core Processor DRAM Controller (rev 06)
00:02.0 VGA compatible controller: Intel Corporation Xeon E3-1200 v3/4th Gen Core Processor Integrated Graphics Controller (rev 06)
00:03.0 Audio device: Intel Corporation Xeon E3-1200 v3/4th Gen Core Processor HD Audio Controller (rev 06)
00:14.0 USB controller: Intel Corporation 8 Series/C220 Series Chipset Family USB xHCI (rev 05)
00:16.0 Communication controller: Intel Corporation 8 Series/C220 Series Chipset Family MEI Controller #1 (rev 04)
00:1a.0 USB controller: Intel Corporation 8 Series/C220 Series Chipset Family USB EHCI #2 (rev 05)
00:1b.0 Audio device: Intel Corporation 8 Series/C220 Series Chipset High Definition Audio Controller (rev 05)
00:1c.0 PCI bridge: Intel Corporation 8 Series/C220 Series Chipset Family PCI Express Root Port #1 (rev d5)
00:1c.5 PCI bridge: Intel Corporation 8 Series/C220 Series Chipset Family PCI Express Root Port #6 (rev d5)
00:1d.0 USB controller: Intel Corporation 8 Series/C220 Series Chipset Family USB EHCI #1 (rev 05)
00:1f.0 ISA bridge: Intel Corporation B85 Express LPC Controller (rev 05)
00:1f.2 SATA controller: Intel Corporation 8 Series/C220 Series Chipset Family 6-port SATA Controller 1 [AHCI mode] (rev 05)
00:1f.3 SMBus: Intel Corporation 8 Series/C220 Series Chipset Family SMBus Controller (rev 05)
02:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller (rev 0c)

Why is my machine appearing to not notice these graphics cards plugged into it?

I have tried:

• Plugging the cards into different PCIe ports on the motherboard.
• Reinstalling the amdgpu-pro card driver.
• In the BIOS settings, changing "Detect Non-Compliance PCI Express Device" from "Disabled" to "Enabled".
• Plugging in just one of the two graphics cards.
• Substituting in different riser cords/plugs.
• Plugging the riser into the PCIe-x16 slot.
• Plugging each card in turn directly into the PCIe-x16 slot.

None of this has affected any change in the output of clinfo or lspci. I am not sure what to try to continue troubleshooting at this point and any help would be much appreciated.

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Here is a picture summary of the plugs from my graphics cards to my motherboard and two power supplies. Power supply #1 is rated at 1300W, power supply #2 is rated at 900W.

• PCIe plug in motherboard to usb cable

• USB cable and 4-pin power cable to graphics card. (Other end of usb cable plugs to PCIe slot on motherboard, other end of 4-pin power cable plugs via SATA to power cord which plugs 5 pins to power supply #1.)

• SATA cable plugs 5-pins into power supply #1. (Other end plugs to 4-pin power cable into card.)

• Power cable 8-pin end plugged into graphics card. (Other end is 6-pins and plugs into power supply #2.)

• Power cable (6-pin end) plugged into power supply #2. (Other end is 8-pins and plugs into graphics card.)