It may be that the disks you are using have changed, even if you are using the same brand. You can check this with DVDIdentifier. Look at the manufacturer id code (MID) on some of the older disks and see if they are the same as these that are failing. Many brands change suppliers quite often, so it is hard to tell what you are buying.
You might have to slow down your burn speed with your current disks, or you might try updating firmware on your burner to see if that will produce better burns.