DIAGNOSIS The thermal print head is dirty or the tape pressure is off. Dirt or adhesive residue on the silver roller (the print head) causes uneven heating, leading to faint spots.
FIX Power off and remove the tape cartridge. You'll see a long, shiny silver metal roller. Get a cotton swab dipped in a tiny amount of isopropyl alcohol (90%+ is best). Wipe the roller gently along its entire length, rotating it with your finger to clean all sides. Let it dry completely for a minute—no moisture allowed. While you're in there, use a dry swab to clean the black rubber pinch roller opposite it. Reinsert the tape and test.
IF THAT DIDN'T WORK The tape cartridge itself might be defective or old. Thermal tape has a shelf life and can degrade. Try a brand-new, sealed cartridge from a known brand to rule out bad tape.
HEADS UP Never use water, window cleaner, or abrasive pads to clean the print head. The alcohol evaporates cleanly and won't cause corrosion. Do this cleaning every few tape changes if you use adhesive-backed tapes, as they leave more residue.