~Thanks Eileen~
~Thanks Cramer~ I washed the glass right after I posted. I do use acrylic coating and workable fixative. Sometimes I don't wait until they are completely dry before I scan them in. So I will keep the screen cleaned.
~Thanks Blue~ I do scan on grayscale, I will check the bit. And if all else fails I will take you up on your offer and send you a copy first.
I have had the scanner for a couple of years. So it could be the scanner. Thanks everyone for your help. ~Robyn~