Riso ink transfer depends on a roller called the anilox — thousands of tiny engraved cells that meter exactly how much ink hits the drum. When ours wore down last spring, every print came out streaky in a way that looked random until we actually pulled the roller and looked at it under light.
The whole repair became its own zine (Process: The Anilox Problem), but the short version is: if your prints are inconsistent in the same direction every time, it’s probably the roller, not the file.