DIAGNOSIS The blade is dull or gummed up with adhesive from the tape backing. This is common after extensive use or with certain third-party tapes.
FIX Open the cutter cover (the sliding door). You'll see the small, half-moon shaped blade. Dip a cotton swab in isopropyl alcohol and clean the blade edge thoroughly. Manually slide the cutter mechanism back and forth a few times to work the cleaner in. Dry it. Test with a piece of scrap tape.
IF THAT DIDN'T WORK The blade may be chipped or misaligned. The blade holder has two tiny Phillips screws. Check they are tight. Do not try to sharpen or replace the blade yourself. The assembly is not user-serviceable.
HEADS UP Never cut laminated or extra-thick "hard" tags with this cutter. It's designed for standard flexible label tape only. Forcing it will damage the mechanism.