download gpu-z read the info of both cards they should be the same model and speed and device id for sli to turn on. also check that both cards are read at right pci bus speed. check your mb info if two cards are plugged in are they running at 8x 8x or one slot 16x and the other 4x.?? also check your game and nvidia v-sink is on. with sli each card is going to be rendering the other frame and then has to be sticked back together.