Burning up grates on my Harmon stoker

Could it be that too much draft is causing the stove to pull air in from the bin. This may in "my theory" cause the fire to be burning to far back on the great?????

Something is sure out of wack. My boiler uses the saem feed and grate system and after 4 years, they are pristine other than alittle discolored. Under the grates is the little swivel that allows fly ash to drop out into the ash bin. Are you sure you have that closed?