cause they were burnt on liteon. all dvds burnt on liteon work best on liteon. i had the same problem back when i had the Sony DRU810A. but then i didnt really burn a lot of dvds on it, and the Sony went bust in like 2 months.
people complain that my dvds dont read, show up as blank or just folder structures with no content in it etc. they work fine on the liteon. hence, i was thinking moserbaer.