DIAGNOSIS The print head is either dirty or the print density setting was accidentally changed. Thermal printing relies on direct, clean contact.
FIX First, check the software setting. In the Nelko app, go to Print Settings. Confirm "Print Density" or "Darkness" is set to the maximum level (usually 4 or 5). If correct, clean the print head. Power off and unplug the P21. Open the tape compartment. Use a cotton swab dipped in >90% isopropyl alcohol. Gently wipe the silver metallic print head bar inside the machine. Let it dry completely for 2 minutes before inserting a new tape cartridge.
IF THAT DIDN'T WORK The thermal print head may be failing. This is a known wear item. Test with a brand new, official Nelko tape cartridge. Third-party tapes can have inconsistent thermal coatings that cause faint prints.
HEADS UP Never use water, window cleaner, or acetone to clean the print head. This will cause permanent damage. Do not touch the print head with your fingers; oils degrade performance.