Chamber temperature is a function of fuel and available air. Make sure you use enough lump, and then that the free flow of air is not obstructed by clogged grates, vents, or even the deflector plate. Adding the plate will always cause a ceramic cooker to heat at a much slower rate, in general, as compared to not having a plate. Any time we use a plate we have to open the vents considerably more than we would without one. Also, we start the fire is several more places, in the beginning or a high-temp cook, so as to shorten the pre-heating time.
The top vent is usually not placed on top of the cooker when we desire temperatures above 500°F or so.
One thing that has been a help to pizza making, to us, is to make three little balls of crumpled aluminum foil, and to place these between the pizza stone and the deflector plate. This has helped to prevent burning crusts while allowing everything to bake to that "just right' doneness.
Also, we made a "stirring stick" by bending a short length of wire coat hanger into an "L" shape. The short end is inserted into the lower vent hole of the cooker and is then pushed up through the holes in the bottom of the fire grate. Pushing up through the lighted lump and wiggling around a bit can make a big difference in how much air is getting to the burning lump. The rapidity of the increase in temperature can be pretty impressive.