I've had mower discharge ports clog before when there is a restrictor metal cross beam at the bottom of the port as grass builds up on this cross beam and sometimes you can't just cut the metal out because the metal strengthens the deck so it doesn't crack in the corners.
I have cut the metal out on mowers and strengthened the port with flat steel and that has fixed the grass clogging in the port as well as stopping any flex in the base now that the lower cross bar is missing.
