Dell Precision R7610 GPU issues?

I've recently received a Precision server from Ebay and was looking to throw in a GPU that I had (GTX 1080). My config came with two PCI risers, one of which appears to be dead. The other is definitely working (I've tested it with a variety of PCI cards, Wireless, a Quadro graphics, etc.) However when I put in the GTX 1080, and connect the 8 pin power connector, I get no device listed in the slot (BIOS shows it as empty). The device clearly has power (fan spinning and on-board LED is lit up), and I have tested it in another computer (It works fine there.)

I was wondering if the 8 pin connector might be the issue, as the Quadro I have has no issue in the same slot, but has no power connector like the GTX 1080? I also tried a GTX 690 in the same slot, but it also didn't show up (It has 2 8 pin slots, which were both connected.) My power supply is rated for 1100W so I doubt the cards are under powered.

I've been doing this testing with the quadro installed and hooked up to my monitor, but that shouldn't affect the devices not being detected at all, right?