Walk Behind Lawn Mowers

I have a husqvarna hu800 awd. I am trying to mulch with it. About every 20 feet or so the mower starts banging and rattling and the it spits out a bunch of un-mulched grass. When done mowing my yard looks awful. Clumps of un-mulched grass all over. The grass was not long at all before cutting. I know that I must mow more frequently if I want to mulch. The under side of the deck has large pockets that the grass builds up in and then it breaks loose and causes the mower make all sorts of noise and then spit out a bunch of grass. This mower was expensive and has been a disappointment from day one. How can I get this thing to mulch properly?