I looked on the Mahoning website but didn't find any 'cut-away' or drawing of the combustion chamber and grate setup. So I'm goiing to have to ask a few questions.
Does the fire box have firebrick on the sides? Is the bottom of the firebox where the air comes from?? Does all the combustion air come up from under the grate or through the loading door or sides??
Coal will only burn well if the air comes up from underneath the grate up through the burning coal. If you have a wood fire going with a solid bed of glowing burning coals then add a layer of coal on top, unless the combustion air is coming up through the bed of coals the fire will not put out much heat. The layer of coal added to the top of the fire may even put out or dampen the fire down due to acting like a blanket blocking air from getting to the fire from above. Only with the air getting to the fire from below will the coal burn well.
You sound like you are shaking the grate correctly, what you describe is what I do on my old MBW coal/wood boiler. But I've noticed that even a very good burning bed of coal is not as hot as a roaring wood fire. I have a hard time getting the water up to temp and keeping it there with coal alone.
I use a blazing wood fire to get the water hot then toss in a bucket of coal to add some slower burning fuel to the firebox. This prolongs the fire and fuel for an extra hour or two.
If you don't have combustion air coming in from under the grate, you could install a combustion blower into the ash drawer or drawer box to blow air up through the grate. My boiler uses this approach for the combustion air.
Hope this helps LsFarm