You might need to increase your RAM voltage to get it to run above 1333MHz. If your memory voltage is currently 1.5v then you could try increasing it a little, up to 1.65v and try increasing the frequency again.
Either way there isn't a significant difference between 1333MHz and 1600MHz in 95% of all applications and that 5% isn't used too often so it's no big deal if you can't go over 1333MHz.
The main reason it is recommended to buy 1600MHz memory is just because it is within the same price range as 1333MHz memory so there is no reason to not get it instead the 1333MHz memory.