At every demonstration that I conduct someone shares a story about their grill struggling to get past 400-500F degrees. I recently had a similar problem in a brand new Kamado Joe grill. No matter what I did with the top and bottom vent, the grill would not get into my prime pizza cooking range of 600F degrees. I figure out that I was experiencing internal blockage that was keeping the grill from getting hot enough. Here are a couple of ideas to remedy this problem:
1) Get a grate in the bottom. I recommend the Kick Ash Basket (I bought mine - and have no financial tie with KAB). Grates allow small chunks of charcoal to drop through rather than block the little air holes of the standard fire grate. The brand new KJ I was using had the standard fire grate and the little holes were getting blocked by charcoal.
2) Be careful with how the heat deflector is placed in the grill. The BGE platesetter allows a lot of airflow around the edges, the KJ deflectors are round and can restrict the airflow. I found that if you put the KJ deflectors up high in the Divide and Conquer system or overlap them by an inch in the middle, you will get more airflow around the outside edges.
3) Ensure that all of the parts of your setup are cleaned out and allow plenty of airflow. For example, the BGE pizza stone for the XL is very large relative to the grill circumference so make sure it is not impeding airflow around the edges.
4) Charcoal. I am charcoal brand agnostic and have had success with almost all brands I have tried. I do make sure that the bags stay dry. Also, make sure that the little pieces don't create a solid matt that keeps the air from flowing into the center of your charcoal.
5) A big fire is hotter than a small one. Start the fire in a couple of spots and get it roaring before closing the lid and using the vents. High temperature means high fuel consumption so go ahead and top up that fire box. The ceramics take time to soak in heat.
Bonus - One other tip I have learned is to monitor the wind. If wind is blowing into the chimney, it is reducing the airflow through the charcoal. Rotate your grill or block the wind.
I hope this is helpful, because pizza really likes a hot oven. The dough rises more rapidly and the dough interior is not overdone. Pizza should be seared like a steak, not baked like a butt.