This is a common problem, and is is normally a function of the firmware for your DVD burner. Each DVD firmware tells the burner what burning strategy to use for each type of recognised disk. If you are using a disk that is not included in your firmware, the DVD burner often selects a strategy for a similar type of disk (e.g. there may be a 4x strategy for the brand you have chosen).
Check the website for your burner - they often list the media codes that are used by your burner. Each disk has its own media code but this is rarely if ever printed on the packagaing of the disk. To get the media code - download the very useful DVDIdentifier freeware program (a search in google will get it in seconds).
It is possible that there is a freeware update (and instructions on how to do it - word of warning: be sure you follow understand and follow the instructions correctly or else the dvd burner may not work properly afterwords if at all. Do not do it if you are not sure - seek more expert advice than I feel qualified to give)
Nero is not usually the problem - equally the disks may well be 8x.