PCIE WiFi card is only recognized in certain slots.

The motherboard is an ASUS P9X79-WS with an E5-2670 and 16GB memory. The video card is an ASUS R9 290. I'm trying to make room for a couple C2050 Tesla cards and the WTXUP TX-AC94360 refuses to be recognized in either x8 (black) slot, even though the LED comes on. It works fine in the x16 (blue) and x4 (white) slots.

If anyone has ideas or explanation, I'd appreciate it.

Please respond if you want more info.