I had a Yard Works that was doing the same thing. It was new and therefore needed no painting under the deck and I cleaned it after every use but it continued to keep clumping the grass.
What I did was remove (not sure what it is called) the small end piecce (plastic) from the mower deck where the cut grass is ejected from. It was getting clogged up there as the opening was too small for the amount of grass that was being cut.
It did not effect the safety of the mower, simply opened up the ejection area and stopped the clumping of grass as it was being expelled.
It worked for me.