Nothing wrong with the pit itself. I assume you close the two block entrance with dirt, glass or cobble (yes, I have glass blocks on my charcoal pit and it works just fine). I wish doors (probably iron only) could be used, but current using any kind of door (wood, iron or trapdoor) will not enclose a charcoal pit sufficiently.
I'm concerned about that one block above the entrance. Apart from that if you aren't getting charcoal, be exceedingly careful that you have a full stack for each column of firewood.