I'm guessing from the your other reply that it isn't going down into the slot far enough to make good contact. If that's the case, on occaision I've had to totally straighten the back metal plate of the card and then re-bend it a bit farther up. Usually right where the 'U' shaped cut is for the screw. If you bend it at the side of the 'U' closest to the card, then cut off the little tab that's normally 'behind' the screw, you can still get a screw in to hold it in place.
On the other hand, if that's not it then you've just wasted another 10 seconds of your life reading this!
