DIAGNOSIS Normal combustion ash accumulation. It must be removed regularly or it smothers the fire and causes temperature problems.
FIX Get a shop vacuum. Let the grill cool completely. Remove the grates, heat baffle, and drip tray liner. You now see the interior bottom pan. Vacuum out all loose ash. Pay special attention to the channel around the fire pot. Then, remove the fire pot itself (usually two bolts or screws). Vacuum inside the fire pot and the small hole underneath it where the hot rod is.
IF THAT DIDN'T WORK If ash is persistently wet and clumpy, you have a grease leak from the drip tray. Ensure your drip tray liner is properly centered and the drain hole at the front of the grill is clear.
HEADS UP Never use a household vacuum. Use a shop vac rated for hot ashes. Embers can linger for over 24 hours.