Try removing both cards, and re-installing them both, to ensure they are both correctly seated.
Also be sure to update your GPU drivers HERE: http://www.geforce.com/Drivers
Check to see if there is a update for your motherboard's BIOS HERE: http://www.gigabyte.com/products/product-page.aspx?pid=3853#bios
Check that both GPU's have sufficient power, and both 6-pin power cables are properly connected (for both cards)
Are you using the correct lanes for SLI?
Hope this helps.