Check the following:

• Are the Print Head and Platen Roller clean?
  A horizontal line can be a result of some dust or dirt accumulated on the surface of the Print Head or Platen Roller.
  This can be corrected by cleaning. For details, see the FAQ: "How to clean the Print Head"or "How to clean the Platen Roller".
• Are the Print Head tension and the ribbon tension adjusted correctly?
  For details, see the FAQ: "The print quality is poor.".