Can't get a bed of coals!

I just built a large trial BBQ. It is constructed of dry stacked concrete block. It is 8' L x 4' D x 5' T. The fire grate is 16" off the ground and the meat grate is 24" above the fire grate. The front and both side are solid stacked block. The back has two openings that are 2' W and 40" tall. they are centered on the back wall. The openings are to clean out the ashes from under the fire grate and to add coals to the fire grate. The fire grate openings have steel doors that can be shut to seal. Above the meat grate there is a two block high back and side walls.

My problem started when I wanted to test it. A wild pig, goat, 2 Boston Butts, and some chicken were prepped. I built my fire using split oak that had dried for about 3 months. After 2 hours of feeding wood I sat back to watch my coal bed develope. After 3 hours I loaded wood again. After 4 hours and another reload I began to realize that it wasn't going to happen. I had suceeded in burning 8 contrator size wheelbarrow loads of oak and did not have 1 inch of coals. I eventually loaded charcoal and had my BBQ but I was unhappy that I couldn't use the wood. After the first two hours I stacked block in front of the ash clean out openings thinking that too much air was getting under the wood but blocking it made no difference. What was wrong.