I've never seen a P&M up close but I've dug into a lot of Heatmaster grated units . I try to trace the airflow all the way from the fan, into the ashpan, up through the grate, through the baffles, and out the chimney. A choke point anywhere will create slow burning which = creosote= less air = creosote... Etc. The problem doesn't go away til the choke point is removed.
But I may not be understanding what your specific issue is.