DIAGNOSIS The self-propel drive belt is likely slipping off its pulley or has stretched and worn out. This is the most common cause of intermittent or failed self-propulsion on this model.
FIX Check and replace the drive belt. First, disconnect the spark plug wire. Tilt the mower on its side (air filter side up). Remove the belt cover. The V-belt runs from the engine pulley to the transmission. Inspect it for cracks, glazing, or looseness. If damaged, replace it (Part Number 29310-VH7-000). Re-route the new belt exactly as the old one was removed, reinstall the cover, and reconnect the spark plug wire.
IF THAT DIDN'T WORK The issue might be inside the transmission itself. The gear oil can become contaminated or low, causing the gears to slip. You'd need to check the oil level via the check/fill plug on the transmission housing and drain/refill if necessary (use SAE 80W-90 gear oil).
HEADS UP When tilting the mower, always ensure the fuel cap is tight and the air filter side is facing up to prevent fuel and oil leaks into the filter and carburetor.