Thanks for the input.
I thought about the moisture, which is why I mentioned that the bin is the standard bin, open at the top. I should have mentioned that I didn't have any trouble storing charcoal this way initially. I should also mention that when I switched from the Kroger brand to the Kingsford, I used fresh charcoal and had the same issue.
Is it possible I have TOO much air flow? I'd think not.
I should also mention I use some moistened hickory for flavor, but again, I have been doing this from the start and have, if anything, been using less of it. I place the wet hickory at the edge of the hot coals.
I'm inclined to try yet another brand of charcoal to see what happens. I had a smaller kettle for a couple of years and had trouble, but I was initially using some off-brand charcoal. When I switched to the Kroger charcoal and started using the chimney, all problems went away.
I will be home late tonight, but perhaps tomorrow morning I can take a photo of the inside of the grill, or perhaps after this weekend (I'm away for the weekend) I can break down the inside. That said, what I saw last night before starting the fire looked perfectly normal. I had some ashes in the kettle, but I moved the air vent back and forth and cleared almost all of it, and then I emptied the ash bucket below.
One thing I do (and have been doing from the start) is the cover the kettle once I've poured the charcoal out of the chimney, let the grill heat up, and then clean the grate once hot. Should I leave the top off for a while to allow more air flow?