Hardware/software info would be helpful. Drive model, OS version, disc types/brands, burning program, etc. And have you done anything to try to solve the problem yet, like trying different discs?
I have a similar problem myself. I can't burn CDs at 24x on my Mac Mini (combo drive). If I try, it'll work just fine, it just won't be fast. It really burns at 8x or 16x speed (I forget which). This happens with both Toast and Disk Utility. I've never found a solution. It can't be the discs, since they work at full speed in other drives. I have an external SuperDrive now, which works fine with all media. I'm fairly certain that this problem didn't exist before I upgraded to Tiger.