The rear panel ports, how do they line up? good? Or do they seem to be a bit high?
Whats the name of the case? Better yet post a link.
If the standoffs are to tall it is possible that the video card is not fully seated because it would hang up where you attach the screw near the back of the case.
Some cases do not use standoffs, linkworld makes some that have little mounds (best way I can discribe it) that the board mounts to and if you would use standoffs the board would sit to high and the rear ports would not line up properly and any cards would be very hard to seat properly.