Slow cooked my first large joint on the weekend, a shoulder of pork (5 pounder).

Put a full chimney of lit charcoal (good quality lumpwood) in the offset smoker and waited for the main grill to come up to temperature (225F).

Barely got past 100F

Ended up creating two heat zones in the main grill instead and cooking it that way, resulting in it getting done an hour or two early, due to it being closer to the heat.

It was perfectly nice, but definitely suffered for the faster cooking time.

What did I do wrong?