Bingo!
Getting 8 - 10 pizzas per firing is about average. And as the oven starts to cool over cooking / time, you have to refire the oven to return the oven to those scorching temps.
Simply pull all the remaining embers over the fire brick / hearth cooking surface (mostly where you are cooking) and add more firewood. Let the oven refire for about 15-20 minutes then resume cooking. Make sure you keep a few logs burning as you continue to cook (this helps keep the oven hot - which will reduce refiring times).
The process of refiring a wood-fired oven applies to all brands of wood-fired ovens. If you want a constant heat (800°+), you are going to need to install a gas burner in your wood fired oven.