DIAGNOSIS The print head energy (heat) setting is too low for the label material you are using. This is a software/driver issue.
FIX The darkness is controlled in your app or computer driver, not on the printer. Open your printing app or driver settings. Locate 'Print Density', 'Darkness', or 'Energy'. Increase the setting by 2-3 increments. For 4x6 shipping labels, the typical optimal range is between 18 and 24 on a 1-30 scale. Print a test. Adjust until the print is sharp and black without excessive background heating.
IF THAT DIDN'T WORK The print head is contaminated. Power off. Dampen a cotton swab with 99% isopropyl alcohol. Gently wipe the silver-colored print head strip (under the lid). Let it dry completely for 60 seconds before closing and printing.
HEADS UP High density settings above 28 will drastically reduce print head lifespan. If you need 28+ for a basic label, the print head may be failing.