I called customer service on this a while back.
According to customer service, the regulator has a "safety mechanism" in it. When that mechanism gets tripped, the regulator limits the amount of fuel it will let go to the burners. The 20lb tank can cause it to trip.
Customer service's suggestion.
Take tank, adapter hose, regulator, griddle, all apart and leave sit for 1 minute - the regulator will reset.
Re-attach in this order.
tank to adapter hose -> adapter hose to regulator -> regulator to griddle.
Then, open the tank SLOWLY. If opened quickly it will trip regulator.
Then light the griddle.
Worked for me. After a few minutes, my griddle temperature was around 550º