I just got a Sapphire rx 480 8gig, my computer won't even recognize it. The fans spins and all that, so i know it has power, but windows 10's device manager doesn't see it.
My computer is less than a year old, it's running 24 gigs of ram and a haswell i7.
Try updating your Motherboard bios. Reseating the GPU. Trying a different PCI-E lane. Trying it in a different computer.
If the above all fail, sounds like it could be faulty.