NEC ND-3500AG Can't Read DVD-R Now

My NEC ND-3500AG can’t read DVD-R now. It burns and checks both DVD+R or DVD-R with Nero Express. It also reads DVD+R. But when a burnt DVD-R
is inserted into the disk-tray, in the pop-up window it is wongly identified
as a blank cdr disk. Of course the same DVD-R is readable by other DVD
What’s wong with it? Could anyone give me some hint?