Wet grass is clogging the deck. The mower can't mulch effectively when the grass is damp.
Fix:
- Stop mowing and wait for the grass to dry completely.
- Clear the accumulated, wet grass clippings from under the mower deck.
If that didn't work:
- You're cutting too much at once. Raise the cutting height to setting 5, 6, 7, or 8.
- Slow your walking pace to reduce the load on the blade.
- Ensure the mulching insert is correctly installed and not damaged.
Prevention: Mow more frequently so you're only removing the top third of the grass blade, and always mow when the grass is dry.