another thing: use different firmware for different discs. I've found that my two 1620s need different firmwares to successfully burn the exact same brand (DAXON AZ2) so keep that in mind. If you get a new batch of media, you might need to change the firmware again for best results.
1620 #1 needs BBFA (for the 1625 burner)
1620 #2 needs B7T9
It can take a lot of testing, but it's worth it when you start to see consistent excellent results