NERO burning ROM- adding more files to already burnt disk - error

Hi I'm using NERO 6.3.1.17 (Dell Inspiron 6400)

Previously I could use the "add more files later" option when burning a disk and add more files later onto the disk. Recently even after selecting that option I'm unable to add files later onto the disk (both DVD and CD). The disk loads displaying the original files but I can't add more files onto the disk despite selecting the "add more files" option prior to burning.


Any advice / suggestions?