Mower will not mulch grass Possible Causes and Corrective Actions

Wet grass
Do not mower wet grass, wait until grass is dry.
Excessively high grass
Mow once at a high cutting height, then mow again at desired height to make narrower cutting path.
Engine speed too low (not applicable in mowers with fixed throttle)
Always run engine at full throttle.
Dull blade
Sharpen or replace blade.