My GTX 680 using EVGA Precision X on both the GPU Clock Offset and the Mem Clock Offset.
Not sure why you are having overclocking issues but I think I may have some insight.
Have you thoroughly read a detailed overclocking guide yet for the 680? My understanding is that overclocking is very different on these cards due to the fact that they will dynamically and aggressively auto overclock. I believe you have to change the constraints on the TDP and possibly add voltage too, if you want to push the card. In other words, the conventional methods of overclocking a video card no longer apply to the 680. See if you can find a good guide or a youtube video to help.