I will make a few recommendations: Remove the inspection panel and clean the area around the heat exchanger tubes. Ash and creostote [if wood was burned] tend to collect around the tubes.
With the stove clean, take a close look at the grates while you move the shaker handle. The grates on Harman handloaders can be moved too far, and the whole fire dumped into the ashpan. So move the handle back and forth, then use chalk to make 'limit lines' on the side of the stove. This will help you from over shaking and dumping your fire, or getting a piece of coal or clinker jammed between the grates.
Also notice that the front and rear grate, next to the front and back wall will not have as much shaking action, as the grates in the middle of the firebox where they move next to another grate. The ash building up on the grate at the front and rear grate causes a loss of combustion air to the fire, and a 'black spot' in the red hot coal bed.
Make yourself a poker out of 2/16" or 1/4" steel rod, bend a 3" or 4" 'L' on one end, and a loop for a handle on the other end. I use political sign wire frame for these pokers
Once you get the dark areas in the coal bed, and shaking hasn't shook down the ash in these areas, use the poker from below, through the ash pan area, and poke between the fingers of the grate, and the ash will fall down and open up the airway.
Keep the loading door vents closed or not open more than a 1/8th turn or so. Use the ashpan spinner vent for the fire.
You will easily get a good hot burn for 12 hours.
Hope this helps.. Greg L