You may have a GPU that doesn't go as high as others. It is the luck of the draw.
However, the FTW edition is already overclocked significantly. Make sure you are comparing it to other cards with a similar factory overclock before assuming that yours won't go as high (don't just look at the core speed offset, look at the actual core speed attained). Also, case cooling is very important with Kepler cards, so make sure you are well ventilated.
Here are a couple of overclocking guides to help with the tweaking:http://www.guru3d.com/article/geforce-gtx-680-overclock-guide/http://hardocp.com/article/2012/04/04/nvidia_kepler_geforce_gtx_680_overclocking_review/