Liteon HD A740GX HDD to DVD Recording problem

I have copied about two 30 minute sections of camcorder footage onto my hard drive. I then tried to copy from HDD to DVD, it did one lot but says there is not enough space for anymore. I know a dvd can hold more than 30 mins worth. what am i doing wrong? do i need to change a setting somewhere? I have now sent 3 e mails to liteon but they have not contacted me in 2 weeks. can anyone here help. thank you.

lee