DIAGNOSIS Wet or tall grass has built up inside the mower deck, impeding airflow and cutting efficiency.
FIX Unplug the mower. Tip it back onto its rear handle, so it rests with the cutting deck facing you. Do not tip it on its side with air vents down. Use a plastic scraper or stiff brush to remove clumps. For stubborn residue, a damp cloth can be used. Ensure the discharge chute and all openings are clear. Let the area dry completely before next use.
IF THAT DIDN'T WORK If debris is fused to the metal, it may have rusted in place. A more aggressive scrub with a wire brush may be needed, but avoid damaging the deck coating.
HEADS UP Always clean the deck after mowing damp grass. This prevents corrosion and maintains proper mulching/discharge function.