This is fairly basic stuff so forgive me if I am quoting something you are already aware of. When I use Express Burn I get a small window offering to erase the disc first when the disc is not blank. The DVD rewritable I am using is a DVD-R (minus R, that is important) I haven't tried a DVD+R yet.
I am also using Windows 7, so previous Window versions or your own PC might not do this automatically. You might have to manually erase the previous write beforehand. If you can't see how, it might be necessary to instal software
that will do this.
Good luck...