Does it actually complete the burn or fail, or is it that it just won't burn at all? If it fails then post the error log as this will help those in the know help you more. If it won't burn at all take us through the steps you go through upto the point where it won't burn. I ask for clarification because to me won't burn means it won't even start the burn process, others may read it as failing part way through the burn.
One obvious thing is to make sure you choose DVD burner. I know it sounds strange, but in Nero my BenQ is listed as both a CD writer and a DVD writer seperately in the drop down menu. I didn't notice this at first and was wondering why it was only giving CD size when I saw the drive listed. It bugged me through re-starting the prog and then the PC, it as only after looking more closely that I saw the seperate listings. Attached is a picture showing what I mean.