Unfortunately, some pellets have a much higher ash content than they should. This may result in larger ash deposits in the firebox which can restrict airflow and cause temperature fluctuations and even soot if the fire gets starved for oxygen.
You should use a shop vacuum to pull the ash out of the firebox every 35-45kg (75-100 lbs) or so of pellets. You may have to do it more often if you use high ash content pellets.
Make sure your grill is cold and there are no remaining embers before cleaning the firebox.