My first guess is your DVD burner does not support Double Layer disc's.
If that's true, you may need to go and buy some Single Layer disc's.
What is the make and model of your DVD burner/drive (hardware)?
Make sure your DVD burner supports the DVD disc format before purchasing blank disc's.
You should fill in your SF System specs - here is one method to get that info: System Info - See Your System Specs
And here's me thinking a dvd is a dvd no matter what brand you purchase.
Unfortunately that's not true