You can try increasing the CPU voltage, but it is likely that you have run into your CPU's multiplier wall. Each individual Sandy Bridge CPU has a certain multiplier that it cannot go past, and this is called it's multiplier wall. No amount of voltage or cooling or settings manipulation will allow it to go higher than its multiplier wall.
For everyday overclocks, it's best to stay below 1.35v or at the absolute maximum 1.4v for CPU longevity. Also, keep the Prime95 load temps below 75ºC.