On my first cook the ash did fill up the space beneath the grates (using briquettes)
I looked at it today and found that the ash was about an inch over the top of the charcoal grate. I think it was smothering the fire. I was using Royal Oak briquettes and it looks like it produces a lot of ash. I may give lump or coshell a try next time.
Try minion method with the lit at the bottom. This doesn't cause the ash to snuff the charcoal as the cool progresses.
Maybe the can in the middle to start the minion. This burns from the middle to the outside during the cook. This is how I always light up my 22 WSM.