Unless you are noticing buffer underruns during the burn, the quote from below may pertain to you. If you ARE noticing buffer underruns during the burn, you may need to defrag your HDD or check your DMA or try not multi-tasking.
Quoted from nero
15. I have a 40x recorder, but I am only able to write at a maximum speed of 32x, sometimes even only 16x write speed. The 40x write speed is not even an option, and yet I know my recorder supports it. What can I do?
If you do not place any media in the recorder until Nero asks you to, are you then able to select the maximum write speed, but once you insert the CD-R media, the write speed drops down? If yes, this is caused by the recorder, determining that the media does not support this write speed, and so forces a lower write speed.
Nero Inc. works with the recorder drive manufacturers very closely. They requested this feature to be able to insure good quality writes. If other programs still allow the full write speed, it's because they override the recorders wishes. This can cause poor or defective writes.
To resolve this issue, you need to have an firmware upgrade of your recorder to support this media at high-speeds, or obtain another brand of CD-R media that the recorder does support at high speeds.(end quote)