DIAGNOSIS The underside of the mower deck is clogged with compacted wet grass, disrupting the airflow needed for proper vacuum lift and discharge.
FIX Clean the deck thoroughly. Disconnect the spark plug. Tilt the mower (air filter up) and use a scraper, putty knife, and hose to remove all caked-on grass and debris from the underside. Pay special attention to the chute and the area around the blade.
IF THAT DIDN'T WORK Check that you're using the correct cutting height settings on all four wheels. An uneven deck (one wheel set higher than others) will cause missed strips. Also, ensure the blade is installed with the cutting edges facing the correct direction and that it's the right part for your deck (Mulching or Standard).
HEADS UP Never use a metal tool like a screwdriver to clear a clog while the engine is running. The blade can strike it and cause catastrophic failure.