X51 R3, cannot overclock to it's max capabilities

I know this computer is quite old comparative to Alienware's newer models and such, but I've been stuck with this i5-6600K for a while now and can't seem to overclock it. I do have the OC Controls setting for overclocking set to ON, but still, haven't seen myself go over the base GHz of 3.5. Any help would be VERY appreciated. Thanks!