If you dont have it, download GPU-Z GPU-Z Video card GPU Information Utility
and make sure everything on your card shows the correct specs.
Make sure the card is fully seated correctly. If it isnt or if it easy to remove, take it out and make sure the slot is clean and make sure it is put back in and seated completely.
If this was an upgrade to the computer and there was previously no card in there, then the system might still be using the integrated graphics processor (IGP). Check in the Device manager to make sure there are no "yellow-warning-triangles". Also, you can check the BIOS to see if it is using your chipset or the Graphics card.
Hope these suggestions help.