Sounds like dust on the negatives, happens to the best of us at times, you can brush, blow, clean the negatives as much as you like but the pesky dust spots will insist on sticking on some negatives,The best thing is a soak in an anti static wetting agent such as tetenal mirosol, or photo flo with a few a few drops of isoprypl alchol in it, that helps to at least reduce the dust on the negatives, but whatever you do it will happen from time to time, the other thing you can try is to get hold of an anti static brush, might help,but it happens to every printer, you need to learn to spot the prints,Richard