I assume you've removed and reseated the card to make sure it is securely and properly seated in the slot.
Is there a chance that there may actually be a setting within the motherboard's BIOS for configuring, enable, disabling, etc., the particular slot.
Does the card itself have a BIOS configuration utility, do you see anything new during POST?
I suppose that it's possible that the card is bad, out of the box. Although rare it's not unheard of. Have you tried it in another system or do you have the option to return it and get another one?