This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
A likely explanation is that the printhead needs to be moved in (toward the big roller) a notch. This is a standard "Paper Thickness" adjustment on many printers. On IW, you do it with a lever to the right of the roller. Another possibility is that the printhead needs cleaning. Be careful what you use to clean a printhead because some solvents can dissolve the mask which lines up the pins. Light oils and gasoline seem to be especially bad. A fine bristle toothbrush plus some standard detergent in warm water or a household cleaner (like Fantastik, etc.) should remove most dust and gunk. Whatever you use, avoid soaking the printhead in anything very long-- i.e. get it reasonably clean and then blow/blot dry. Changing settings on the DIP with the factory settings which "users should leave alone" may have some effect on firing of the bottom pins. I don't know. Probably, you would want to try everything else first. ----------------------------