As the fallen leaves build up deep under large shade trees or if they are wet, a single pass of a mulching mower is often not sufficient to sufficiently shred them into small pieces. Sometimes several passes over the area will be required. (For best effect, mow "across the grain" on each pass.)
For really deep pile of leaves, there are a couple of techniques to try: you can spread the leaves out (thin the layer) into other areas of the lawn before shredding. Alternately, a leaf blower can be used post-shred to help "even out" the mulch over-burden across the lawn and/or into landscape beds.
A deep pile of leaves can be "attacked" by raising up the front of the mower and coming down on top of the pile, over and over, to pre-shred. Then perform a normal pass with the mower.
Other possible factors? The height of the mower deck (thus the blade height) might need to be adjusted lower to keep leaf shreds under the mower and in the blade path longer. Is the mulch gate (deck door) closed? Perhaps the blades have not been sharpened recently?