I use Nero 6 a lot, but have never used a DVDRW disk. I know that when burning a CD and you add more stuff on you must make sure mulitsession is ticked. Its on the final page that says Burn. Otherwise it will finalise the disk and you can't add more on it.
I assume its the same with a DVDRW.
And I think I should mention that DVDRW disks are not good enough for long term storage. You're better using either CDs or DVDs not the rewritable equivalents and create a multisession disk.