DS7 can't follow a pen either, however when I watch his eyes reading they track just fine. We had taken him to a neuro-opthamologist and she as alarmed by how poorly he did tracking on command, but watched him read and then said "Obviously his eyes are fine." It's something about doing it "on command" that DS can't handle. I think it has to do with his developmental coordination disorder and planning movements, but spontaneously in the real world he is Ok. I'm not saying that I think DS's eyes are for sure "ok" (and I have no idea about your DD) but wanted to give my experience.
