Could also be that it hits a filename of a certain length or a filename with a character that it does not like and it just stops scanning the directory structure any further.
I have not done any test, but the DVD drive does not seem to like for instance the Microsoft Joliet extension for Navigation DVDs. So you might try burning a DVD with the filesystem set to ISO9660, file length set to ISO Level 2 (max 31 chars) and the character set to ISO 9660.
Again this is just a guess. For all I know it indeed does not like DL (Dual Layer) media.