Garvin, one thing is too make sure the whole grill is heat soaked. If you have an IR thermometer check the temp of the stone. The grill thermometer may read 500F but the stone may be much much lower. Get the pizza on as fast as possible and don't open the lid any more than you have to do get the pizza on and no peaking, don't let that heat escape. Hope this helps some.
The good news is that you were able to eat this experiment, keep on working at it.