Nero 6 burn problem



Hi. My problem is this- I burn some movies into a dvd using nero 6, as
dvd data-because that way I can put more stuff on one dvd- I don’t mind watching it on the pc only. Lately, for some reason, after I burn (the data dvd) the computer won’t recognize the dvd, behaves as if the dvd is empty. when I try to burn the stuff on the same dvd again,then and only then nero informs me that the disc is not empty. Any idea whats the problem?
thank you for your time