I believe those scanners have a dual adf, which would mean one of the 2 sensors is acting up, or is covered.
So you should be able to have a look here (step 5) for the one that does side 2.
My DM3220 suddenly stopped scanning both sides of the document. The top side of the document comes out black while the bottom side copies perfectly. I have Windows XP and everything worked just fine until now. The scanner was purchased in April of 2016. TIA