It seems that you have addressed a few questions, and since the chimney seems to draw fine, the baro is working, and the controls move appropriately, the firebox if full, your shakedown technique is OK,
that leaves one other issue: THE FUEL you are trying to burn.
Did you by chance pick up that pipe of Newport Coal Stoker Scott dropped in an abandoned driveway off Rt361? Just kidding, but you may be 'flattening out the learning curve' if the answer is this simple.
That boiler should be easy to run and keep perfect temps. It is one of the best hand feds out there. Remember this: a stoker eliminates all this part of the curve, but we all learn as we burn.
I Just refurbed a Crane Coal Cooker #44 and fired it up yesterday. Small but well built for a purpose: making heat!!!!!