The reason could be your CD-Rs are rated for 40X and will not support 52X. Any limitations are usually the media not the drive. The drive and software will burn only as fast as the media can handle.
Crucial's rite .. What u can do is ...Use the nero toolkit to check the actual speed the media can handle.
Also ... u can turn off the smart burn option ..if u want nero to burn only at the speed u specify n' not automatically adjust to the speed of the media..