Hi mrweaver. Is the 'ash and coal that doesn't make it to the pan' from near the front of the firebox, or along the sides of the firebox?? The ashes build up along the edges and at the front edge of the first grate on the SF fireboxes.
The grates shake the ash down pretty good in the middle of the firebox because there are adjacent grates rocking as well. While your firebox is empty and cold, move the shaker handle through the same amount of movement you normally use and watch the grates. In the center of the firebox where two grates are side by side, one edge lifts, and the adjacent edge drops, so say a 3/4" tilt of a grate is enough to open a gap between adjacent grates. This is enough shaking action to agitate the ash and get it to drop through to the pan.
But at the front of the firebox, the front of the first grate just lifts the ash/coal and it doesn't develope enough agitation or open enough of a gap for the ash to fall through to the pan. The same happens at the edge of the grates next to the firebrick.
So the ash doesn't get shook down into the pan and stays on top of the grate, blocking airflow to the coal, so the coal doesn't burn well in these areas.
A Forum member near me has a Harman SF150. He was having a problem with no fire along the front and sides of the firebox. We make up a small poker from a piece of 1/4" diameter steel rod. We bent the end 4 or 5 inches at a 90* angle. Using this poker from underneath the grates, accessing through the ashpan door, we poked up through the gaps in the grates along the front of the firebox and along the edges. A lot of ash was dislodged and fell into the ashpan. Once the ash was dislodged, the fire burnt well along the front and sides of the firebox.
He has to use his poker about every couple of weeks to keep the airways open and the coal burning all the way across the firebox.
While you have the firebox empty watch the grates and figure out how far you can move the handle without opening the grates too far and dumping the fire into the ashpan. You may need to shake your fire more aggressively if your coal has a lot of ash in it.
Hope this helps.. Greg L