Gigabyte 1080 Ti not found by Asus motherboard

Please help,

I've upgraded my video card from a 980 ti to a 1080 ti, but my motherboard does not detect the new card.

When I insert the new card I get no display. The leds and fans do turn on but the bios switches to the iGPU. Forcing pcie does not work. Inserting the old card works perfectly fine. I've also tested the 1080 ti in a second system and confirmed the card is not faulty (it immediately switched to the 1080 ti on boot).

Specs:

• New card: Gigabyte Aorus 1080 Ti XTREME

• Old card: Zotac 980 Ti AMP Xtreme

• Motherboard: Asus Maximus VIII Extreme (yeah i know I'm gullible for the word extreme)

• PSU: Corsair AX860i

• CPU: i7 6700K

Any help would be greatly appreciated!

UPDATE:

I tried the following (still no luck) :

• Updating bios to the latest version

• HDMI, DVI, DisplayPort... none work

• All 4 PCIe slots card didn't work in any

• Connecting the new card in my mini itx build works (with Corsair SF450) psu).

• Using that 450 psu to power my main system with the OLD card works, using that small psu to power my main system with the NEW card does not work.

• Running DDU

UPDATE 2 (01/07/17):

• I'm currently running the card on an Asus STRIX Z270G with a i7 7700K (also tested the 6700K just to be sure) and this is running perfectly fine.

• I sent back my motherboard but Asus returned it unrepaired noting that it tested fine on their side.

• The card still isn't recognized in my Maximus 8 though, so I have still no clue what is going on.