The next time you have your firebox cleaned out, take a look at the grates.. you will see the fingers pointing at each other.. then work the shaking lever, note how the fingers move apart, leaving a big gap for a hard chunk of coal to get into and jam the grates..
I recommend to new Harman product owners to to the above 'inspection' of the grates. then using a piece chalk, or if it would work a few pieces of tape, and mark the maximum travel of the shaker bar on the side of the stove or in your case boiler. This will help you to not open the grates too wide when shaking down the ash, and causing a jam. It's a technique learned by all Harman hand fired owners..
A quick comment about shaking and coal.. If the coal is not being burnt so hot that it is fusing the ash into clinkers, [temps in the ash above 2700*], then the coal should be burning down to be just a powdery ash, like chunky oatmeal or granola cereal. IF the coal is good quality coal, with low ash content, and very little shale in it, this is what you will have, just gritty, powdery ash..
If the coal has a high ash content and has lots of unburnable shale in it, then you will have lots of frustration trying to shake this unburnable stuff through the grates.. The only way is grind this stuff up with repeated shaking with a heavy load of coal pressing down on the layer of ash, so the grates get to grind down the hard ashes.. sort of like putting more weight on a file when filing a piece of metal or wood, it cuts better with more weight applied..
When I had my 'Big Bertha' boiler running in handfired mode, I had lots of frustration with the grates I had installed in this firebox.. they didn't shake or rock, they slide back and forth, like a file trying to shave off the bottom of the ashes.. it didn't work well at all with most coal. The combination of chunks of ash staying behind after shaking, and the high fire temperatures I was running, created some spectacular clinkers.. the size of phone books..
What I finally had to do, was build a fresh fire each week, the first few days it burnt well., but as un-shakable [un grindable] ash accumulated, the fire got weaker, the air to the fire was being blocked by clinkers, and by the end of the week, the fire could not keep the boiler up to temp.. so I let the fire go out, and dug out the huge clinkers and started with a clean firebox.. Sometimes I was able to get under the big clinker on day 4 or 5, and pull it up the side of the firebox, leaving the fire mostly intact, and was able to go another week.. but this was rare..
Hope the above helps you climb your learning curve with your boiler.