When you started shaking, do you push the lever all the way back, or pull it all the way forward? With the 2 grate system on the Harman, the 2 grates move in the same way, if you go to far when shaking it will dump the contents in between the grates, you won't be able to move the lever back to center position. You should shake using short, chopping strokes, back and forth.
I've had instances where large clinkers or unburnt coal gets stuck between the grates, if I move the lever a little further it usually drops into the ash pan.
Pushing the lever all the way is useful when emptying the stove.
Shaking a Harman hand-fired takes practice, it's part of the learning curve associated with this stove.