The map is huge, and there are a lot of people in Hogwarts. They probably assumed they kept missing Ginny in all the names, on the off chance they looked for her at all. Voldemort probably showed up as Tom Riddle, and his real name isn’t common knowledge. The likely just assumed it was a student that talked to Quirrell a lot. I think polyjuice potion is the only thing that can trick the map. He showed up as Bartimus Crouch that one time as he had run out of polyjuice potion, and was running to get some more. Peter Pettigrew… the dormitories are probably stacked on top of each other, with the beds overlapping, so they assumed it was just an older student.
Comic Credit: Julia Lepetit