Like Andy said, you must have the air going through the burn screen so it can evacuate the burned pellets so check the burn pot / screen and make sure it is all the way to the left as you're facing the unit.
Insufficient airflow will cause the burn pot to fill up with ash even using the cleanest burning pellets. Take the 6 screws out of the end panel that has the supply fan and pull the entire panel off. GO SLOW as there may not be a lot of slack in the wires that supply the fan motor. (Hopefully you've unplugged the unit by now.) The wires connecting the fan motor are spade terminals and they can be easily lifted off thus making the end panel completely unattached.
What you are looking for is to make sure the wires are bundled up out of the way. The fan needs a straight shot to be able to blow enough air into the burn pot. If your wires are near the bottom of the control cabinet, get some wire or zip ties and and suspend them up and out of the way.
If all of the vent holes are wide open, try propping the lid open an 1/8 to a 1/4 inch. (I've just clipped a wooden clothespin to the bottom left corner of the lid when I wanted to relieve some heat (and airflow). I too had trouble with branch creek pellets in my original CS570 factory condtion. With the wires up out of the way and one EXTRA vent hole, even Branch Creek pellets will burn fine and blow the ash out of the pot. You will go through a few more pellets with increased airflow but your combustion problems should go away.
American BBQ Systems model All Star, CS-570 & TG-300 pellet grills, Broilmaster P3, and lastly, Superfast Red Thermapen