How old are the cobs?
i've got 4 MM cobs that I've purchased in the last 2 months 4 more on the way.
I always run a pipe cleaner through them after they have cooled off and when I am done a smoke I always take the small pick on my czech tool and make sure there is no debris at the very bottom of my cob below the air hole.
I've noticed on all of my cobs that there is a small gap beneath the air hole and the bottom of the pipe. Don't know why, there just is. I always make a point to get the small bit of tobacco debris that lodges in there after a smoke, out. My reasoning is that if I don't, it might build up (not cake) and give off an odor.
But other than that my cobs are my best smokers. You will need a flashlight to see what I mean. I always use a small flashlight to check the bottom after each smoke.
Chris in Red Deer, Alberta