Multisession Disk

I own a copy of Nero Express 6. When i try to burn a multisession disk (by checking the checkbox next to “allow files to be added later”), all seem to work fine. But when i insert the disk later (lets say after 5 days) for further burning, and i select “data disk -> add files”, the program says that the disk is already full (as if it were finalized). Why does that happen? Am i doing something wrong?

Thanks in advance!