What kind of blank CDs are you using? I have noticed the same thing with my 52327S; it will only burn older CMC media (Imation 16x, Imation 32x, Philips 40x, etc.) at 24x. It burns TDK 40x (Ritek) and Fuji 48x (Taiyo Yuden) at 52x perfectly, though!
edit I'm using firmware QS0A, but the same thing happens with QS08 (came with my drive) and QS09, too.