Roxio Does Not Allow Additions to DVD

[RexxSysProg]

I copy data files to a DVD, selecting the option "I want to use the disk for more recording". After Roxio verifies the copies, it says its done. The data on the DVD is good. When I re-open the "project" to append more files, Roxio says that the disk is appendable with 960MB used. But when I tell Roxio to add to files it says the DVD is full, even though there is over 3GB of space left (and Roxio just said it's appendable). Apparently it "closed" the DVD instead of leaving it open for more files, even though that was the option chosen and it even says that when I re-load the DVD.
What is the problem?

XP 5.1 SP2; Roxio 9.0

 

[RexxSysProg]

Just to clarify the problem: I used the DATA -> DATA DISC function (I did not use DATA-> CREATOR CLASSIC function.)