Losing sessions on multisession DVDs

I downloaded and installed NeroLINUX 2.1.0.1 on my Ubuntu 6.06 system. However, whenever I burn something on a multisession DVD I get to see only the files burnt in the last session.
What can cause this ?

Before adding new files to your last session, do you import the last session first? You should then see them (old files) in blue.