Try another slot and see if it gets detected. There's always the remote chance the second card is dead. I ran into that 3 weeks ago - wasted 6 hours operating on the assumption that the PCIe GEN3 slots were acting up. It wasn't until I gave up and tried the card in my rig at home that I find out that the card was DOA.
Nope, I can get the card to work if I move it to different slots.
OK, here is a "key" to this image:
Red := I have been told the card will not work here. Have not tried.
Blue := Open slot, I have been told that it should work.
Green := The card is currently working.
So, I thought it might be an x4 or x16 issue... you think that matters?