When you click "Print" in a design software like BarTender, ZebraDesigner, or even Microsoft Word, your computer sends a high-level command. The printer, which speaks a different language (usually a command language like EPL, ZPL, or ESC/POS), cannot understand this command directly. The intercepts this data, converts it into a language the printer understands, and instructs the print head on exactly where to apply heat to create the image on the thermal paper.
The wrong driver is selected (e.g., using a standard LaserJet driver for a thermal printer). Fix: rh-40 thermal printer driver